Enhanced coupling strength grating having a cover layer

ABSTRACT

The present invention includes an optical waveguide with a grating and a method of making the same for increasing the effectiveness of the grating. In one example, the grating is at least partially covered by a liner layer disposed on at least a portion of a grating; and a cover layer disposed on the liner layer, wherein a first material selected for the core and ridges and a second material selected for the liner layer are selected to provide a difference in the index of refraction between the first and second material that is sufficient to provide a contrast therebetween.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 14/479,039 filed on Sep. 5, 2014, now U.S. Pat. No.10,371,898, which claims priority based on U.S. provisional ApplicationNo. 61/874,162, filed Sep. 5, 2013. The contents of which isincorporated by reference in its entirety.

STATEMENT OF FEDERALLY FUNDED RESEARCH

This invention was made with Government support under Agreement No.HR0011-08-9-0001 awarded by DARPA. The Government has certain rights inthe invention.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to the field of electromagneticradiation, and more particularly, to an apparatus and methods to enhancethe coupling strength of electromagnetic radiation coupled by gratingsin optical waveguides.

BACKGROUND OF THE INVENTION

Without limiting the scope of the invention, its background is describedin connection with optical waveguides.

High index contrast silicon/silicon dioxide optical waveguides canradiate 60 to 100% of the light propagating in the waveguide from agrating etched into one surface of the waveguide in short distances—inabout 10 to 20 microns (or about 20 to 40 grating periods). The periodof these radiating gratings is typically at or near the second Braggcondition, meaning that the grating period is equal, or approximatelyequal to the wavelength of the radiated light propagating in thewaveguide (or the free space wavelength λ₀ divided by the effectiveindex). At or near the second Bragg condition, there can be asignificant second-order Bragg in-plane reflection. This often undesiredreflection could be eliminated by tilting the radiated light (e.g., withan appropriate choice of the grating period) sufficiently off of theaxis normal to the laser surface, or by the addition of one or moreadditional slits (grating grooves or ridges) appropriately spaced awayfrom the coupler grating. The additional slit(s) serve as a partiallyreflecting mirror and by destructive interference cancel the in-planereflection.

However, generating light on a silicon wafer is problematic. There arenumerous other semiconductors, often alloys of elements in columns IIIand V (III-V compounds—which include GaAs and InP alloys) and columns IIand VI (II-VI compounds) of the periodic table that commonly generatelight and from which semiconductor light-emitting diodes (LEDs) andsemiconductor lasers are fabricated. Being able to couple light out of ashort section of a waveguide formed in a compound semiconductor opticalwaveguide has several advantages including: 1) optimum coupling tosingle- and multi-mode optical fibers; 2) optimum coupling to siliconphotonic waveguides; and 3) economic gains by reducing the real estateused by the grating coupler.

One example is taught in U.S. Pat. No. 7,006,732, issued to Gunn, III,et al., entitled, “Polarization splitting grating couplers.” Briefly,this patent teaches a polarization splitting grating coupler (PSGC) thatconnects an optical signal from an optical element, such as a fiber, toan optoelectronic integrated circuit. The PSGC is said to separate areceived optical signal into two orthogonal polarizations and to directthe two polarizations to separate waveguides on an integrated circuit.

Another example is taught in U.S. Pat. No. 7,068,887, also by Gunn, III,et al., entitled, “Polarization splitting grating couplers.” Again, apolarization splitting grating coupler (PSGC) is said to connect to anoptical signal from an optical element, such as a fiber, to anoptoelectronic integrated circuit, and is capable of separating areceived optical signal into two orthogonal polarizations, and directsthe two polarizations to separate waveguides on an integrated circuit.The two separated polarizations can then be processed, as needed for aparticular application by the integrated circuit. The PSGC can alsooperate in the reverse direction.

Another example is taught in U.S. Pat. No. 6,760,359, also by presentinventor (Evans), and is entitled, “Grating-outcoupled surface-emittinglasers with flared gain regions.” Briefly, a laser system is taught thatincludes a laser diode with an active region and reflectors at bothends. An outcoupling aperture is located between the reflectors tocouple light out of the device through the surface. The gain regionincreases in width as it nears the outcoupling aperture.

High index contrast silicon/silicon dioxide optical waveguides canradiate 60 to 100% of the light propagating in the waveguide from agrating etched into one surface of the waveguide in short distances—inabout 10 to 20 microns (or about 20 to 40 grating periods) at awavelength of 1550 nm. The term “light” and “optical” refer toelectromagnetic waves that extend to wavelengths shorter (ultraviolet)and longer (infrared) than light visible to the human eye. Presentlysemiconductor LEDs and lasers span the wavelength range from about 0.3microns to many tens of microns.

Despite many advances in the field, a need remains to enhance couplingof electromagnetic radiation due to gratings.

SUMMARY OF THE INVENTION

The present invention describes devices and methods for enhancing thecoupling strength of gratings formed in dielectric waveguides of alltypes, including optical waveguides formed in compound semiconductorwaveguides and optical waveguides formed in glasses and used for fiberoptics. Such Enhanced Coupling Strength (ECS) gratings in compoundsemiconductor waveguides can match the performance of gratings in highindex contrast silicon/silicon dioxide optical waveguides. Inparticular, ECS gratings couple power in very short distances comparedto common gratings in semiconductor waveguides.

The present invention applies to gratings in all dielectric waveguidesfor any region of the electromagnetic spectrum. Although the examplesshown in this patent primarily relates to gratings used to couple lightinto or out of a waveguide (commonly referred to as gratings withperiods that are at or near the 2^(nd) order), this invention applies togratings of all orders. In particular this patent applies to gratingswith periods that are at the first order. Such first order gratings areused for feedback in distributed feedback (DFB) lasers and as integratedmirror reflectors in distributed Bragg reflector (DBR) lasers.

The present invention applies to all wavelengths (including microwaveand millimeter waves) of the electromagnetic spectrum that is guided bydielectric waveguides or waveguides that contain dielectrics. Thisinvention also applies to making the gratings in high index contrastsilicon/silicon dioxide waveguides more effective, too. In addition,this invention applies to gratings used for coupling light into and outof optical waveguides, to gratings used for reflecting light, togratings used for transmitting light, and to gratings used fordeflecting light. Being able to efficiently couple light out of (orinto) a short section of a waveguide formed in a compound semiconductoroptical waveguide has several advantages including: 1) optimum couplingto single- and multi-mode optical fibers; 2) optimum coupling to siliconphotonic waveguides; and 3) economic gains by reducing the real estateused by the grating coupler. In the following discussion, forsimplicity, calculations are made for waveguides with a one-dimensionalcross section with TE polarized modes, although the results for TMpolarized modes differ by very little. Further, the invention can beused to enhance the coupling strength of high-index contrast Si/SiO₂gratings and applies equally to waveguides with two-dimensional crosssections. The waveguide grating couplers obey reciprocity and thereforecan be used to couple light into or out of optical waveguides with equalefficiency. The described method of enhancing the coupling strength ofgratings can also be used to reduce losses at discontinuities in opticalwaveguides.

In one embodiment, the present invention includes an enhanced couplingstrength grating, comprising: a substrate (12); a core (14) disposed onthe substrate (12); one or more ridges (16) and one or more grooves (17)formed on the core (14), wherein the one or more grooves (17) areadjacent to, or between the one or more ridges (16), wherein the ridges(16) and grooves (17) form a grating (19); a liner layer (18) disposedon at least a portion of a grating cycle; and a cover layer (20)disposed on the liner layer (18), wherein a first material selected forthe core (14) and ridges (16) and a second material selected for theliner layer (18) are selected to provide a first difference in the indexof refraction between the first and second material that is sufficientto provide a contrast therebetween. In one aspect, the liner layer (18)is disposed on at least one of: the bottom (28) of the groove (17); oneor more sidewalls (22, 24) of the ridges (16); on the top (26) of theone or more ridges (16); two or more liner layers (18) in the groove(17); or on the sides of the ridges (16) that do not have a top (26). Inanother aspect, the liner layer (18) selected from one or more of thefollowing optional configurations: (a) the liner layer (18) is notcontiguous; (b) the liner layer (18) is disposed on a first sidewall(22), a second sidewall (24), or both the first and second sidewalls(22, 24) of the ridges (16); (c) the liner layer (18) is defined furtheras one or more liner layers (18) that are contiguous and that follow thecontour of the ridges (16) and the grooves (17); (d) the liner layer(18) is not contiguous, wherein the liner layer (18) is defined furtheras being substantially parallel to a bottom of the one or more grooves(17), and the non-contiguous layers are separated by one or more coverlayers (30); (e) the liner layer (18) is defined further as two or moreliner layers (18) that are contiguous and that follow the contour of theridges (16), and each of the two or more liner layers (18) are separatedby one or more cover layers (20); (f) the liner layer (18) is disposedon one or more tops (26) of the ridges (16), one or more grooves (17)between the ridges (16), or both the top (26) of the ridges (16), andthe grooves (17) between the ridges (16); (g) the liner layer (18) isdisposed on a first sidewall (22), one or more tops (26) of the ridges(16), and one or more grooves (17) between the ridges (16), to providean effective blazed grating; (h) the liner layer (18) is disposed on afirst sidewall (22) and one or more tops (26) of the ridges (16); or (i)the liner layer (18) is disposed on one or more first sidewall (22) orsecond sidewall (24) of one or more waveguiding structures for gratingcoupling (inward or outward).

In another aspect, the liner layer (18) is disposed on a high indexcontrast Si/SiO2 waveguide to further enhance the performance of thegrating. In another aspect, a third layer for the cover layer (20) isselected to provide a similar index contrast or second difference in theindex of refraction between the cover layer (20) and the liner layer(18) as the contrast or first difference in the index of refractionprovided between the core (14) and the liner layer (18). In anotheraspect, the liner layer (18) is selected from at least one of SiO, SiO₂,MgF₂, Al₂O₃, HfO₂, Ta₂O₄₋₅, Sc₂O₃, ZrO₂, TiO₂, CaF₂, ThF₄, ZnS, ZnSe,polymers, and silicon nitride. In another aspect, the liner layer (18)comprises a variable thickness to provide at least one of varying thestrength of the coupling, an effective variable duty cycle, an effectivevariable grating depth, a Gaussian profile in a radiating couplergrating, or a near-Gaussian profile in a radiating coupler grating. Inanother aspect, the liner layer (18) is selected to provide at least oneof an optical loss or an optical gain.

In another aspect, the optical waveguide is at least one of adistributed Bragg reflectors (DBRs) or a distributed Bragg deflectors(DBDs). In another aspect, the optical waveguide is defined further ascomprising at least two ECS gratings to make an edge-emitting DBR laser;one ECS grating and one regular DBR grating to make an edge-emitting DBRlaser; two ECS gratings with a straight ECS outcoupler grating to make asurface-emitting laser; ECS grating and one regular DBR grating with astraight ECS outcoupler grating to make a surface-emitting laser; twoECS gratings with a “fan-out” ECS outcoupler grating to make asurface-emitting laser; one ECS grating and one regular DBR grating witha “fan-out” ECS outcoupler grating to make a surface-emitting laser; twoECS gratings with a standard grating outcoupler grating to make asurface-emitting laser; or one ECS grating and one regular DBR gratingwith a standard grating outcoupler grating to make a surface-emittinglaser; one or more ECS grating output couplers with low back reflectionon both ends to make a surface-normal coupled semiconductor opticalamplifier (SOA) or optical gain block; or one or more ECS gratings orregular DBR gratings configures as a mirror with high reflectivity andanother ECS grating as an output coupler to make a surface-emittingreflective semiconductor optical amplifier (RSOA) or an optical gainblock; a hybrid external cavity laser and tunable laser using SOA orRSOA with ECS grating output couplers integrated with a waveguide orfree space wavelength control optics; or an enhanced grating for highdensity and low loss integration of III/V laser sources for siliconphotonic interconnects.

In another aspect, the optical waveguide is an enhanced grating on mesaswith gratings etched into the sides of the mesas; an enhanced gratingfor grating-assisted directional couplers; enhanced grating for multipleresonant distributed feedback lasers; or an enhanced grating formultiplying resonant distributed Bragg reflector lasers; an enhancedcoupling strength (ECS) grating in optical fibers for sampling ordetecting light in optical fibers by grating outcouplers; an enhancedgrating in optical fibers for (1) sampling or detecting light in opticalfibers by ECS grating outcouplers operating near the second order Braggcondition; (2) sampling or detecting light in optical fibers by ECSgrating outcouplers operating as distributed Bragg deflectors; (3) tocouple light into optical fibers; a curved, enhanced grating to makeunstable resonator semiconductor lasers; an enhanced grating to reducethe etch depth for the placement of distributed Bragg reflector gratingsin semiconductor lasers, which results in simplified processing for DBRlasers; an enhanced grating to reduce the etch depth for the placementof distributed Bragg reflector gratings in photonic devices, whichresults in simplified processing for photonic devices; and enhancedgratings to reduce the etch depth for the placement of gratings inphotonic devices, which results in simplified processing for photonicdevices; or an enhanced grating to reduce the etch depth for theplacement of coupling gratings in photonic devices, which results insimplified processing for photonic devices. In another aspect, thegrating (19) comprise a period that is equal to about the wavelength ofthe light propagating in the optical waveguide to produce an outcouplingin about 10 to 50 grating cycles. In another aspect, the grating (19)comprise a period that is equal to about one half the wavelength of thelight propagating in the optical waveguide, resulting in an in-planereflectivity of up to about 100% in about 5 to 50 grating cycles forlight in a typical III-V waveguide.

In another aspect, the cover layer (20) is defined further as anamorphous or crystalline cover layer selected from at least one of Si,GaAs, AlGaAs, InP, InGaAsP, GaN, AlGaN, InGaAsPSb, GaP, spin polymers,other column IV, column III-V, or column II-VI semiconductors. Inanother aspect, the cover layer (20) is defined further as an amorphousor crystalline high index layer or an amorphous or crystalline low indexcover layer and the cover layer (20) is deposited or formed by at leastone of sputtering, vapor phase deposition, plasma enhanced chemicalvapor deposition, vapor phase epitaxy, molecular beam deposition,molecular beam epitaxy, spin-on, or atomic layer deposition or epitaxialgrowth over dielectrics through openings in the dielectric to exposedepitaxial material. In another aspect, the cover layer (20) is definedfurther as an amorphous low index cover layer selected from at least oneof silicon nitride, polymer, SiO, SiO₂, MgF₂, Al₂O₃, HfO₂, Ta₂O₄₋₅,Sc₂O₃, ZrO₂, TiO₂, CaF₂, ThF₄, ZnS, ZnSe, and other dielectrics. Inanother aspect, the cover layer (20) is defined further as an amorphouslow index cover layer deposited by at least one of sputtering, vaporphase deposition, plasma enhanced chemical vapor deposition, vapor phaseepitaxy, molecular beam deposition, molecular beam epitaxy, atomic layerdeposition, or by a spin-on processes. In another aspect, the coverlayer (20) converts a grating from a grating region that does notsupport a bound-mode to a grating region that does support a bound-mode.In another aspect, the optical waveguide is defined further ascomprising a non-grating transition waveguide, wherein the non-gratingtransition waveguide comprises a high index cover layer or a low indexcover layer that converts a high loss discontinuity between thewaveguide and the transition waveguide to a low loss discontinuity, andmay optionally further comprise a second contrasting cover layer. Inanother aspect, the cover layer (20) when applied over a liner layerconverts a grating from a grating region that does not support abound-mode to a grating region that does support a bound-mode. Inanother aspect, the ridges (16) of the grating (19) extend above thecore layer (14). In another aspect, the thickness of each of the corelayer (14), grating (19) liner layer (18), and cover layer (20) arevaried to optimize the ratio of upward coupled radiation to downwardcoupled radiation or in the upwards or downwards direction. In anotheraspect, a period is selected that couples radiation at an anglesufficiently tilted from a surface-normal to reduce or eliminatesecond-order in-plane Bragg reflection. In another aspect, the opticalwaveguide further comprises one or more additional grooves (17) orridges (16) each with enhanced coupling strength gratings to provide apartially reflecting mirror that reduces or cancels a second-orderin-plane Bragg reflection by destructive interference. In anotheraspect, the optical waveguide further comprises one or more additionalgrooves (17) or ridges (16) to provide a partially reflecting mirrorthat reduces or cancels a second-order in-plane Bragg reflection bydestructive interference. In another aspect, the optical waveguidefurther comprises one or more additional grooves (17) or ridges (16)that are not covered by at least one of the liner layer (18) or coverlayer (20) to provide a partially reflecting mirror that reduces orcancels a second-order in-plane Bragg reflection by destructiveinterference. In another aspect, the optical waveguide further comprisesone or more additional grating ridges that are not covered by at leastone of a liner layer or a cover layer.

In another aspect, the index of refraction of the liner layer (18) isthe range of ˜1.3 to ˜1.7, 1.7 to ˜2.2, ˜2.2 to ˜3, or ˜3 to ˜3.8. Inanother aspect, the cover layer (20) is at least one of amorphous orcrystalline silicon and is defined further as a high index cover layerthat is compatible with silicon processing. In another aspect, thegrating period of the ridges (19) is adapted for use with wavelengths inthe range of 0.1 to 0.4, 0.4 to 1.0, 0.5 to 1.1, 0.6 to 1.1, and greaterthan 1.1. In another aspect, the selection of the materials for theridges (16) is adapted for use with wavelengths in the range of 0.1 to0.4, 0.4 to 1.0, 0.5 to 1.1, 0.6 to 1.1, and greater than 1.1. Inanother aspect, the core (14) and the ridges (16) are unitary. Inanother aspect, the grating forms at least a portion of an opticalwaveguide.

In another embodiment, the present invention includes a method of makinga grating comprising: depositing on a first portion of substrate (12), acore (14) on the substrate (12), a superstrate (42) on the core (14),and a photoresist (44) on the superstrate (42); etching the superstrate(42) to (or into) the core (14); etching through a grating mask (48)formed on the core (14) to form the grating (48); removing the gratingmask depositing a liner layer (50) on the grating 48, wherein a firstmaterial is selected for the core (14) and ridges (16) and a secondmaterial selected for the liner layer (50), wherein the first and secondmaterials are selected to provide a first difference in the index ofrefraction sufficient to provide a contrast therebetween; and depositinga cover layer (52) on the liner layer (50). In one aspect, the grating(48) comprises at least a portion of a waveguide (40). In anotheraspect, the grating (48) is defined further as comprising on at leastone of: a bottom (28) of a groove (17); one or more sidewalls (22, 24)of one or more ridges (16); a top (26) on the one or more ridges (16);two or more liner layers (18) in the groove (17); or one or moresidewalls (22, 24) on the one or more ridges (16) that do not have a top(26). In another aspect, the liner layer (18) selected from one or moreof the following optional configurations: (a) the liner layer (18) isnot contiguous; (b) the liner layer (18) is disposed on a first sidewall(22), a second sidewall (24) or both the first and second sidewalls (22,24) of the ridges (16); (c) the liner layer (18) is defined further asone or more liner layers (18) that are contiguous and that follow thecontour of the ridges (16) and the grooves (17); (d) the liner layer(18) is not contiguous, wherein the liner layer (18) is defined furtheras being substantially parallel to a bottom of the one or more grooves(17), and the non-contiguous layers are separated by one or more coverlayers (30); (e) the liner layer (18) is defined further as two or moreliner layers (18) that are contiguous and that follow the contour of theridges (16), and each of the two or more liner layers (18) are separatedby one or more cover layers (20); (f) the liner layer (18) is disposedon one or more tops (26) of the ridges (16), one or more grooves (17)between the ridges (16), or both the top (26) of the ridges (16), andthe grooves (17) between the ridges (16); (g) the liner layer (18) isdisposed on a first sidewall (22), one or more tops (26) of the ridges(16), and one or more grooves (17) between the ridges (16), to providean effective blazed grating; (h) the liner layer (18) is disposed on afirst sidewall (22) and one or more tops (26) of the ridges (16); or (i)the liner layer (18) is disposed on one or more first sidewall (22) orsecond sidewall (24) of one or more waveguiding structures for gratingcoupling (inward or outward).

In another aspect, the liner layer (50) is disposed on a high indexcontrast Si/SiO2 waveguide to further enhance the performance of thegrating. In another aspect, a third material for the cover layer (20) isselected to provide a similar index contrast or second difference in theindex of refraction between the cover layer (20) and the liner layer(18) as the contrast or first difference in the index of refractionprovided between the core (14) and the liner layer (18). In anotheraspect, the liner layer (50) is selected from at least one of SiO, SiO₂,MgF₂, Al₂O₃, HfO₂, Ta₂O₄₋₅, Sc₂O₃, ZrO₂, TiO₂, CaF₂, ThF₄, ZnS, ZnSe,polymers, and silicon nitride. In another aspect, the liner layer (50)comprises a variable thickness to provide at least one of varying thestrength of the coupling, an effective variable duty cycle, an effectivevariable grating depth, a Gaussian profile in a radiating couplergrating, or a near-Gaussian profile in a radiating coupler grating. Inanother aspect, the optical waveguide is at least one of a distributedBragg reflectors (DBRs) or a distributed Bragg deflectors (DBDs). Inanother aspect, the optical waveguide is defined further as comprisingat least two ECS gratings to make an edge-emitting DBR laser; one ECSgrating and one regular DBR grating to make an edge-emitting DBR laser;two ECS gratings with a straight ECS outcoupler grating to make asurface-emitting laser; ECS grating and one regular DBR grating with astraight ECS outcoupler grating to make a surface-emitting laser; twoECS gratings with a “fan-out” ECS outcoupler grating to make asurface-emitting laser; one ECS grating and one regular DBR grating witha “fan-out” ECS outcoupler grating to make a surface-emitting laser; twoECS gratings with a standard grating outcoupler grating to make asurface-emitting laser; or one ECS grating and one regular DBR gratingwith a standard grating outcoupler grating to make a surface-emittinglaser; one or more ECS grating output couplers with low back reflectionon both ends to make a surface-normal coupled semiconductor opticalamplifier (SOA) or optical gain block; or one or more ECS gratings orregular DBR gratings configured as a mirror with high reflectivity andanother ECS grating as an output coupler to make a surface-emittingreflective semiconductor optical amplifier (RSOA) or an optical gainblock; a hybrid external cavity laser and tunable laser using SOA orRSOA with ECS grating output couplers integrated with a waveguide orfree space wavelength control optics; or an enhanced grating for highdensity and low loss integration of III/V laser sources for siliconphotonic interconnects.

In another aspect, the optical waveguide is formed into at least one ofan enhanced grating on a mesa waveguide with gratings etched into thesides of the mesa; an enhanced grating for grating-assisted directionalcouplers; an enhanced grating for multiple resonant distributed feedbacklasers; or an enhanced grating for multiplying resonant distributedBragg reflector lasers; an enhanced grating in optical fibers forsampling or detecting light in optical fibers by grating outcouplers; anenhanced gratings in optical fibers for (1) sampling or detecting lightin optical fibers by grating outcouplers operating near the second orderBragg condition; (2) sampling or detecting light in optical fibers bygrating outcouplers operating as distributed Bragg deflectors; (3) tocouple light into optical fibers; a curved, enhanced gratings to makeunstable resonator semiconductor lasers; an enhanced grating to reducethe etch depth for the placement of distributed Bragg reflector gratingsin semiconductor lasers, which results in simplified processing for DBRlasers; an enhanced grating to reduce the etch depth for the placementof distributed Bragg reflector gratings in photonic devices, whichresults in simplified processing for photonic devices; and enhancedgrating to reduce the etch depth for the placement of gratings inphotonic devices, which results in simplified processing for photonicdevices; or an enhanced grating to reduce the etch depth for theplacement of coupling gratings in photonic devices, which results insimplified processing for photonic devices. In another aspect, thegrating (19) comprise a period that is equal to about the wavelength ofthe light propagating in the optical waveguide to produce an outcouplingin about 10 to 50 grating cycles. In another aspect, the grating (19)comprise a period that is equal to about one half the wavelength of thelight propagating in the optical waveguide, and up to about 100%in-plane reflectivity occurs in about 5 to 50 grating cycles for lightin a typical III-V waveguide. In another aspect, the cover layer (52) isdefined further as an amorphous or crystalline cover layer selected fromat least one of Si, GaAs, AlGaAs, InP, InGaAsP, GaN, AlGaN, InGaAsPSb,GaP, other column IV, column III-V, column II-VI semiconductors, SiO,SiO₂, MgF₂, Al₂O₃, HfO₂, Ta₂O₄₋₅, Sc₂O₃, ZrO₂, TiO₂, CaF₂, ThF₄, ZnS,ZnSe, polymers, and silicon nitride. In another aspect, the cover layer(52) is defined further as an amorphous or crystalline high index layeror an amorphous or crystalline low index cover layer and the cover layer(52) is deposited or formed by at least one of sputtering, vapor phasedeposition, plasma enhanced chemical vapor deposition, vapor phaseepitaxy, molecular beam deposition, molecular beam epitaxy, spin-onprocess, or atomic layer deposition or epitaxial growth over the linerlayer through openings in the liner layer to exposed epitaxial material.In another aspect, the cover layer (52) is defined further as anamorphous low index cover layer deposited by at least one of sputtering,vapor phase deposition, plasma enhanced chemical vapor deposition, vaporphase epitaxy, molecular beam deposition, molecular beam epitaxy, atomiclayer deposition, or by a spin-on processes. In another aspect, thecover layer (52) converts a grating from a grating region that does notsupport a bound-mode to a grating region that does support a bound-mode.In another aspect, the optical waveguide is defined further ascomprising a non-grating transition waveguide, wherein the non-gratingtransition waveguide comprises: (a) a high index cover layer or a lowindex cover layer that converts a high loss discontinuity between thewaveguide and the transition waveguide to a low loss discontinuity, (b)a high index cover layer or a low index cover layer that converts a highloss discontinuity between the waveguide and the transition waveguide toa low loss discontinuity and may optionally further comprise a secondcontrasting cover layer; or (c) a tapered waveguide; or (d) aninverse-tapered waveguide. In another aspect, the ridges (16) of thegrating (19) extend above the core layer (14). In another aspect, thethickness of each of the core layer (14), grating (19) liner layer (18),and cover layer (20) are varied to optimize the ratio of upward coupledradiation to downward coupled radiation or in the upwards or downwardsdirection. In another aspect, a period is selected that couplesradiation at an angle sufficiently tilted from a surface-normal toreduce or eliminate second-order in-plane Bragg reflection. In anotheraspect, the optical waveguide further comprises one or more additionalgrooves (17) or ridges (16) each with enhanced coupling strengthgratings to provide a partially reflecting mirror that reduces orcancels a second-order in-plane Bragg reflection by destructiveinterference. In another aspect, the optical waveguide further comprisesone or more additional grooves (17) or ridges (16) to provide apartially reflecting mirror that reduces or cancels a second-orderin-plane Bragg reflection by destructive interference. In anotheraspect, the optical waveguide further comprises one or more additionalgrooves (17) or ridges (16) that are not covered by at least one of theliner layer (18) or cover layer (20) to provide a partially reflectingmirror that reduces or cancels a second-order in-plane Bragg reflectionby destructive interference.

In another aspect, the optical waveguide further comprises one or moreadditional grating ridges or teeth that are not covered by a coverlayer. In another aspect, the index of refraction of the liner layer(50) is the range of ˜1.3 to ˜1.7, 1.7 to ˜2.2, ˜2.2 to ˜3, or ˜3 to˜3.8. In another aspect, the cover layer (52) is at least one ofamorphous or crystalline silicon and is defined further as a high indexcover layer that is compatible with silicon processing. In anotheraspect, the grating period of the ridges (16) are adapted for use withwavelengths in the range of 0.1 to 0.4, 0.4 to 1.0, 0.5 to 1.1, 0.6 to1.1, and greater than 1.1. In another aspect, the selection of thematerials for the ridges (16) is adapted for use with wavelengths in therange of 0.1 to 0.4, 0.4 to 1.0, 0.5 to 1.1, 0.6 to 1.1, and greaterthan 1.1. In another aspect, the core (14) and the ridges (16) areunitary.

In another embodiment, the present invention includes an opticalwaveguide comprising: one or more cladding layers deposited on one ormore core layers, wherein the cladding layers comprise a refractiveindex that is lower than the refractive index of the core layers,wherein the optical waveguide is further defined as comprising a gratingetched into at least one of a cladding layer, the core, or the claddingand core layers. In one aspect, the refractive index of the claddinglayers and core layers are ˜1 to 2. In another aspect, the opticalwaveguide is further defined as comprising a grating etched into atleast one of a cladding layer, the core, or the cladding and corelayers. In another aspect, the optical waveguide is further defined ascomprising a grating that has a liner layer disposed thereon and theliner layer has a cover layer, wherein the cover layer has at least oneof a lower-index of refraction than the liner layer or a higher index ofrefraction than the liner layer. In another aspect, the opticalwaveguide is further defined as comprising a grating that has a coverlayer disposed thereon and the index of refraction of the cover layer is˜1 to ˜2, or ˜2 to 4. In another aspect, the optical waveguide isfurther defined as comprising a grating with a period that is equal toabout the wavelength of the light propagating in the waveguide toproduce a coupling in 5 to 50 grating cycles. In another aspect, theoptical waveguide is further defined as comprising a grating with aperiod that is equal to about the wavelength of the light propagating inthe waveguide to produce a coupling in 5 to 50 microns for light in atypical III-V waveguide at a free space wavelength of about 1.5 or even1.55 micron.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the features and advantages of thepresent invention, reference is now made to the detailed description ofthe invention along with the accompanying figures and in which:

FIG. 1 shows a typical silicon photonics waveguide of the prior art.

FIG. 2 shows a typical DFB waveguide of the prior art.

FIG. 3 shows a typical DBR waveguide of the prior art.

FIGS. 4A and 4B shows a periodic boundary (4A) is equivalent to astraight boundary (4B) with a periodic surface currentJ(z)=−iε₀ωηW(ε₁−ε₂)cos(Kz)E(W,z).

FIG. 5 shows a sketch of a waveguide that includes an Enhanced CouplingStrength grating with a thin, low-index liner and high index cover ofthe present invention. FIG. 5 is a modified version of the DBR waveguideshown in FIG. 3 using the present invention.

FIG. 6 shows a sketch of a waveguide that includes an Enhanced CouplingStrength grating with a fiber-like planar glass index waveguide, theliner layer index is large compared to the index of the core and coverlayer indices.

FIG. 7 shows an Si/SiO₂ waveguide with a liner layer and a cover layerof the present invention.

FIG. 8 shows a sketch of regions I, II and III of a grating with a linerlayer and a cover layer for use with the present invention.

FIG. 9A shows the index profile for the waveguide structure shown inFIG. 5 for a liner layer having a thickness of 5, 10 and 25 nm as shownin the figure legend with the grating layer replaced by the square rootof the average relative permittivities in grating regions I, II and III.

FIG. 9B shows the resulting intensity profiles for the fundamental modefor the index profiles in FIG. 9A for a liner layer having a thicknessof 5, 10 and 25 nm as shown in the figure legend.

FIG. 10A shows an index profile of the ECS grating coupler waveguide(FIG. 5) with a fixed core thickness of 0.2 microns, a grating thicknessof 0.3 microns and a variable thickness for the high index cover layer.FIG. 10B shows the grating confinement factor as a function of thethickness of the high index cover layer.

FIG. 11A shows an index profile of the ECS grating coupler waveguide(FIG. 5) with a variable core thickness, a grating thickness of 0.2microns and a thickness for the high index cover layer of 0.2 microns.FIG. 11B shows the grating confinement factor as a function of the corethickness. FIG. 11C shows the field intensity plots for core thicknessesranging from 0.1 to 0.5 microns as shown in the figure legend.

FIGS. 12A to 12C shows: FIG. 12A is an index profile of the ECS-gratingcoupler waveguide (FIG. 5) with a fixed core thickness of 0.2 microns, avariable thickness for the grating layer and a fixed thickness of 0.1microns for the cover layer. FIG. 12B shows the grating confinementfactor as a function of the thickness of the grating layer. FIG. 12Cshows a field intensity plots for grating thicknesses ranging from 0.1to 0.5 microns as shown in the figure legend.

FIGS. 13A to 13C show: FIG. 13A shows a sketch of a waveguide thatincludes a grating with liner and cover layers. FIG. 13B shows a gratingconfinement factor as a function of grating depth for core thicknessesvarying from 0.1 to 0.4 microns as shown in the figure legend. FIG. 13Cshows the field intensity plots for a fixed grating depth of 0.1 micronsfor core thicknesses varying from 0.1 to 0.4 microns as shown in thefigure legend.

FIG. 14A shows a sketch of a waveguide that includes a grating withliner and cover layers. FIG. 14B shows plots of the magnitude of thefield intensity at the lower boundary of the grating for a normalizedcore thickness of W/λ for index differences between the core and thesubstrate ranging from 0.1 to 0.5 as shown in the figure legend.

FIG. 15 contains multiple plots of the field intensities for thewaveguides shown in FIGS. 1, 2, 3, 5, and 7. FIG. 7 is an ECS version ofFIG. 1. In these examples all waveguides have the same core thicknesses(0.2 microns) and grating depths (0.1 microns). The ECS DBR waveguideand the ECS Si/SiO₂ waveguide each have a liner thickness of 5 nm. Inthe right two panels, a liner is depicted above the grating and core.

FIG. 16 is a plot of the normalized peak attenuation as a function ofliner thickness for the ECS grating coupler waveguide shown in FIG. 5with a core thickness of 0.2 microns, a grating depth of 0.1 microns, agrating period of ˜0.47 microns and a cover thickness of 0.15 microns.

FIG. 17 is a cross section of the laser waveguide region (Section 1,shown in Table 3), a first transition region (Section 2), a secondtransition region (Section 3), and the grating outcoupler region(Section 4, shown in Table 4). The results of the overlap integral inthe laser waveguide region of the field intensity in Section 2 (98.9%,96.2%) and Section 3 (96.8%, 93.2%) with the field intensities inSection 2 and 3 are also indicated, with and without a liner. Theoverlap integral of the field intensity in the laser waveguide regionwith the field intensity of Section 4 (94%) is shown only for the caseof a liner and an amorphous Si layer.

FIGS. 18A and 18B show the details of the grating after etching anddeposition of the liner and cover layers for structures that may havethe top portion of the grating in InP and the bottom portion of thegrating in InGaAsP, corresponding to grating fabrication in a waveguidethat corresponds to the cross-section shown in section 2 of FIG. 17.FIG. 18A shows grating ridges consisting of InP (index=3.16492), thelayer below the grating is InGaAsP (index=3.35110), the liner layer issilicon dioxide (index=1.46) and the cover layer is amorphous silicon(index=3.476). FIG. 18B shows etching the grating further into the SCHlayer requires adding a fourth “average relative permittivity” layer.FIG. 18C shows grating ridges consisting of n=1.4 material over a corematerial with index n=1.5. FIG. 18D shows etching the grating furtherinto the core layer requires adding a fourth “average” relativepermittivity layer.

FIGS. 19A to 19C show the following: FIG. 19A shows plots of the fieldintensities and index profiles in the laser waveguide region (Section 1of FIG. 17, □ index profile and □ field plot); in Section 2 of FIG. 17with no liner or cover layer (× index profile and × field plot, 93%overlap); and in Section 2 of FIG. 17 with a liner layer and anamorphous Si cover layer (Δ index profile and Δ field plot, 99%overlap). FIG. 19B shows plots of the field intensities and indexprofiles in the laser waveguide region (Section 1 of FIG. 17, □ indexprofile and □ field plot); in Section 3 of FIG. 17 with no liner orcover layer (× index profile and × field plot, 73% overlap); and inSection 3 of FIG. 17 with only an amorphous Si cover layer (Δ indexprofile and Δ field plot, 93% overlap); and in Section 3 of FIG. 17 witha liner and amorphous Si cover layer (∘ index and ∘ field plot, 97%overlap). FIG. 19C shows plots of the field intensities and indexprofiles in the laser waveguide region (Section 1 of FIG. 17, □ indexprofile and □ field plot); and in Section 4 of FIG. 17 with a liner andcover layer (× index profile and × field plot, 94% overlap).

FIG. 20 shows the normalized reciprocal wavelength (Λ/λ) as a functionof normalized longitudinal propagation constant (β/K) and normalizedattenuation (αΛ) calculated using the Floquet Bloch approach for the ECSgrating coupler shown in FIG. 19C and Table 4, assuming a 25 nm linerthickness. The points (1), (2), (3) and (4) correspond to a range ofwavelengths (see FIG. 21) if the grating period is assumed to beconstant. (Conversely, the points (1), (2), (3) and (4) correspond to arange of grating periods if the wavelength is assumed to be constant.)

FIG. 21 shows the fraction of incident power radiated down (⋄), fractionof incident power radiated up (*), fraction of incident power reflectedbackwards (•), fraction of incident power transmitted forward (▪);fraction of incident power radiated both upwards and downwards (▴) andthe sum of the fractions of incident power reflected, transmitted andradiated (♦) as a function of wavelength (assuming a fixed grating witha 50% duty cycle) for the waveguide shown in FIG. 19C and Table 4. Thedeviation of the total power plot (♦) from unity indicates that themaximum error of the Floquet Bloch analysis is about 10% at a wavelengthof 1550 nm and drops to less than 3% for wavelengths greater than 1560nm. The points (1), (2), (3) and (4) in FIG. 21 correspond to the samefour points shown in FIG. 20. These calculations assumed a gratinglength of 19.5 microns and a grating period of 0.4882 microns. FIGS. 22Ato 22D show: FIG. 22A shows the intensity distribution in the 5 QWgrating waveguide region shown in FIG. 19C and Table 4 at point (1) inFIGS. 20 and 21, which is the peak of the attenuation curve and occursat a wavelength of 1565 nm. At point (1) the total radiated power is˜20% and the total reflected power is ˜80%, which illustrates that thewavelength of maximum attenuation is not the desired wavelength formaximum outcoupled radiation. FIG. 22B shows the intensity distributionin the 5 QW grating waveguide region shown in FIG. 19C and Table 4 atpoint (2) in FIGS. 20 and 21, which corresponds to the wavelength (1592nm) at which the maximum total power is radiated (74%) and the reflectedpower is ˜10%. FIG. 22C shows the intensity distribution in the 5 QWgrating waveguide region shown in FIG. 19C and Table 4 at point (3) inFIGS. 20 and 21, which corresponds to the wavelength (1610 nm) at whichthe total power radiated is ˜67% and the power reflected is low (˜5%).FIG. 22D shows the intensity distribution in the 5 QW grating waveguideregion shown in FIG. 19C and Table 4 at point (4) in FIGS. 20 and 21,which corresponds to the wavelength (1630 nm) at which the total powerradiated is ˜60% and the power reflected is very low (less than 1%). Thewhite lines within the frame of the colored plots outline the ridges andgrooves of the grating. For wavelengths sufficiently far away from aBragg resonance, the intensity distribution within a grating cycleremains constant along the direction of propagation.

FIGS. 23A to 23H shows various embodiments of ECS gratings with acontinuous liner layer of the present invention. The grating profile canbe rectangular (FIG. 23A), sinusoidal (FIG. 23B), sawtooth (FIG. 23C),sawtooth with a flat region in the groove (FIG. 23D), blazed sawtooth(FIG. 23E), blazed sawtooth with a flat region in the groove (FIG. 23F),trapezoidal (FIG. 23G), and dovetail (FIG. 23H).

FIG. 24 shows an ECS grating with a segmented liner layer only on thebottoms and tops of the grating of the present invention.

FIG. 25 shows an ECS grating with a segmented liner layer only on thesidewalls of the grating of the present invention.

FIG. 26A shows an ECS grating with the grooves of the grating filledwith liner material of the present invention. FIG. 26B shows an ECSgrating with the grooves of the grating partially filled with linermaterial of the present invention. FIG. 26C shows an ECS grating withthe grooves of the grating overfilled with liner material of the presentinvention.

FIG. 27 shows an ECS grating with horizontal multi-layers of liner andcover layers of the present invention. The core of the waveguides can below index or high index, with a contrasting liner layer and/or coverlayer.

FIG. 28 shows an ECS grating with multi-layers of liners and coverlayers on all surfaces of the grating of the present invention.

FIG. 29 shows an ECS grating with asymmetric liner layer of the presentinvention resulting in an effective blazed grating.

FIG. 30 shows an alternate configuration of an asymmetric liner ECSgrating of the present invention.

FIG. 31A is a sketch of an ECS grating coupler region on a simplewaveguide such as shown in FIG. 5. A gray circular beam is shownindicating light radiated from the ECS grating coupler at an angle tothe normal to the surface. In the following figures, the same gray coloris used to indicate photoresist. FIG. 31B is a top view of an ECSgrating coupler integrated with a DBR laser and shows how a narrow (0.1to 5 microns) mesa waveguide can be expanded in a very short distance toproduce an ECS grating with dimensions of about 10 microns by 10 micronsfor efficient coupling to fiber optics and/or other grating waveguidecouplers. FIG. 31C is a side view of the integrated ECS grating couplerof FIG. 31B. Although FIG. 31C shows that all the gratings (for examplea short period grating and a long period grating) are formed at the samevertical level of the structure and that all gratings have the samedepth, in general gratings can be formed at different levels and withdifferent depths. FIG. 31D shows an alternative form of the waveguidemesa in which the gratings are positioned on the top of the mesa, FIG.31E shows the waveguide on the top surface just outside the mesa, and31F shows the waveguide positioned along the sides of the mesa. FIG. 31Gshows that the gratings can occur on both top surfaces or on both topsurfaces and the sides of the mesa.

The following figures also show one possible sequence (out of numerouspossible sequences) of how an ECS grating coupler could be fabricated.

FIG. 32 shows the initial step in fabricating an ECS grating couplerrequires covering the wafer containing the waveguide with photoresist.The left, middle and right view corresponds to the end, side and topview of a section of the wafer.

FIG. 33 shows the second step in fabricating an ECS grating coupler andshows the definition in photoresist of a mesa- or ridge-waveguide. Theleft, middle and right view corresponds to the end, side and top view ofa section of the wafer.

FIG. 34 shows the third step in fabricating an ECS grating couplerdefines the mesa waveguide by etching away a section of the superstrate.The left, middle and right view corresponds to the end, side and topview of a section of the wafer.

FIG. 35 shows the fourth step in fabricating an ECS grating couplerprotects the mesa with photoresist and defines a grating in photoresist.The photoresist grating can be formed by several procedures such asholography, e-beam writing, or standard lithography. The left, middleand right view corresponds to the end, side and top view of a section ofthe wafer.

FIG. 36 shows the fifth step in fabricating an ECS grating coupler isreplicating the photoresist grating into the waveguide by any of severaletching procedures such as wet chemical etching, plasma etching, ionbeam etching, reactive ion etching, chemically assisted ion beam etchingor inductively coupled plasma etching. The left, middle and right viewcorresponds to the end, side and top view of a section of the wafer.

FIG. 37 shows the sixth step in fabricating an ECS grating coupler isdepositing a liner material over the exposed grating. The left, middleand right view corresponds to the end, side and top view of a section ofthe wafer.

FIG. 38 shows the final step in fabricating an ECS grating coupler isthe deposition of a cover layer over the liner material. The left,middle and right view corresponds to the end, side and top view of asection of the wafer.

DETAILED DESCRIPTION OF THE INVENTION

While the making and using of various embodiments of the presentinvention are discussed in detail below, it should be appreciated thatthe present invention provides many applicable inventive concepts thatcan be embodied in a wide variety of specific contexts. The specificembodiments discussed herein are merely illustrative of specific ways tomake and use the invention and do not delimit the scope of theinvention.

To facilitate the understanding of this invention, a number of terms aredefined below. Terms defined herein have meanings as commonly understoodby a person of ordinary skill in the areas relevant to the presentinvention. Terms such as “a”, “an” and “the” are not intended to referto only a singular entity, but include the general class of which aspecific example may be used for illustration. The terminology herein isused to describe specific embodiments of the invention, but their usagedoes not delimit the invention, except as outlined in the claims.

The present invention includes an optical waveguide with a grating and amethod of increasing the effectiveness of the grating. The opticalwaveguide includes at least one core layer surrounded by cladding layerswith (at least slightly) lower indices of refraction. The claddinglayers are sometimes referred to as a substrate layer and a superstratelayer. The present invention solves a number of problems in the art.First, a new class of optical waveguide grating couplers is formed incompound semiconductor materials including GaAs and InP alloys. In thepresent invention, the core layer(s) of most semiconductor materialshave a relatively high index of refraction (2.5 to 4.0 or so) with one(see description of DBR structure, FIG. 3) or both (see description ofDFB structure, FIG. 1) cladding layers having a slightly lower(difference in index can be 0.001 to 0.5, 0.01 to about 0.4, 0.1 toabout 3.0, 0.001 to 0.3, 0.1, 0.2, 0.3, 0.4, 1.0, 1.5, 2.0, 2.5, 3.0)index. In this case a “low index liner layer” (over the grating formedin the high index (core or cladding) material) covered with a high indexcover layer results in an enhanced grating.

The present invention solves the problem of the lack of availability ofstrong gratings in III-V semiconductor waveguides. The present inventionalso allows the making of strong gratings on low index glass waveguides.The present invention also allows for one or more liner layers and/orcover layers, or just cover layers, on the grating that reduce losses atwaveguide transitions. Finally, the liner/cover layers of the presentinvention addressed the need to make a strong silicon photonics gratingeven stronger. Thus, in certain embodiments, a “high index liner layer”(over the grating formed in the low index (core or cladding) material)is covered with a low index cover layer that also results in an enhancedgrating. In another embodiment, the present invention can use alow-index liner material and a high index cover layer material when thecore and grating ridges are high-index. In another embodiment, thepresent invention can use a high index liner material and a low indexcover material when the core and grating ridges are low index.

The present invention is based on the need to have a short, efficientgrating (e.g., the high index contrast Si/SiO₂ waveguide grating of FIG.1, which works with the low index contrast waveguides of FIGS. 2 and 3.In the present invention, the liner layer thickness is typically a verysmall fraction of a wavelength, e.g., 5 to 50 nm for an SiO₂ layer at awavelength of 1550 nm. Generally, the cover layer is thicker and againdepending on wavelength. The skilled artisan will recognize that areduced index difference between the grating ridge and the linermaterial decreases the strength of the grating coupler. However, thereare times when the user may not need or desire the maximum gratingstrength (or enhancement), so the choice of liner material and covermaterial allows for a range of grating performance. Because an immediateapplication of this invention is to the area of silicon photonics, theexamples below will assume a wavelength of 1550 nm. However theinvention is applicable to all wavelengths in the electromagneticspectrum.

As used herein, the term “contrasting layer” or “contrasting” whenreferring to the difference between the core layer, grating ridge, linerlayer and/or the cover layer describes the use of layers with differentindices of refraction. The indices of all layer and the thicknesses ofall layers determine the intensity distribution within all layers of thewaveguide.

As used herein, the term “mesa” or “mesa waveguide” refers to astructure that provides lateral (two-dimensional) waveguide confinementin the direction perpendicular to the direction of light propagation inthe waveguide. The term mesa waveguide is a broad term and as usedherein includes any type of lateral optical confinement such as providedby a ridge-waveguide, a slab-waveguide, a buried-hetero-structurewaveguide or a waveguide formed by disordering.

For high index core waveguides (such as III-V waveguides) amorphousand/or crystalline silicon can be used as the high index cover layer,and provides the advantage of being generally compatible with siliconprocessing. Generally, silicon only has low losses at wavelengthsgreater than about 1.1 microns, which is useful for silicon photonicapplications. As taught hereinbelow, numerous compounds can be used atshorter wavelengths and longer wavelengths, e.g., when usingsemiconductor lasers that emit from about 0.4 to tens of microns. Thepresent invention can also be used in common applications for gratingsat wavelengths in the 0.6 to 1.1 micron (and greater) range. Onematerial for use at wavelengths in the 0.5 to 1.1 micron range is GaP,which can be deposited, e.g., using standard sputtering systems. Oneadvantage of GaP is that it has very low losses for wavelengths greaterthan 0.5 microns. Sputtering is a very common process and relativelyinexpensive compared to molecular beam epitaxy (MBE) or metalorganicvapour phase epitaxy (MOVPE), which can also be used with the presentinvention. Commonly, crystalline and amorphous silicon are used insilicon photonics applications, which makes them useful as a cover layerfor wavelengths greater than about 1.1 microns. In one example, GaP canbe used as a cover layer for wavelengths greater than about 0.5 microns.

The present invention is different from a high index contrast Si/SiO₂waveguide. In this case, the Si core has a high index (about 3.5), butboth cladding layers are low index (about 1.5). A standard grating insuch a Si photonics waveguide can be very efficient. However a gratingin such a Si photonics waveguide can be made even more efficient usingthe present invention of applying a low index SiO₂ liner layer with ahigh index amorphous Si layer as shown in column 7 of Table 1.

As used herein, the term “Enhanced Coupling Strength” (ECS) Grating isused to describe ECS gratings that can be first order (in planereflection only), 2^(nd) order (outcoupling and sometimes in planereflection), or higher order (multiple outcoupling angles and in planereflections). While certain embodiments of the present invention showcalculations near the 2^(nd) order Bragg conditions, the “enhancement”works equally well for all grating orders.

For example, another class of optical waveguides of the presentinvention can be formed in low index glasses or polymers, such as thoseused to make optical fibers. The core layer(s) have a low index ofrefraction (1 to about 2) with cladding layers of slightly lower(difference in index of 0.001 to 0.5, 0.01 to about 0.4, 0.1 to about1.0, 0.001 to 0.3, 0.1, 0.2, 0.3, 0.4, 1.0, or so) index.

The skilled artisan will recognize that during processing the index ofrefraction for a particular material at a particular location may varyslightly from calculated value, the present invention includes suchvariability. In this case variations in the indices of the liner andcover layers still result in an enhanced grating.

In one example, the present invention includes the development of, e.g.,an InP based semiconductor laser (emitting at a wavelength ˜1550 nm)integrated with an ECS grating coupler with a near-field spot sizediameter of approximately 10 to 15 microns (in the direction along thewaveguide). Such a grating coupler is needed because it matches thegrating couplers fabricated in high index contrast silicon/silicondioxide optical waveguides which can radiate 80 to 100% of the lightpropagating in the waveguide in such short distances—about 10 to 20microns. However, previous to this invention, optical waveguides formedin compound semiconductor materials including GaAs and InP alloysrequire a grating that is hundreds or even thousands of microns long toradiate the same amount of light.

Silicon/silicon dioxide optical waveguides are fundamental to thedeveloping field of Silicon Photonics. However, generating light on asilicon wafer is problematic. There are numerous other semiconductors,often alloys of elements in columns III and V (III-V compounds—whichinclude GaAs and InP alloys) and columns II and VI (II-VI compounds) ofthe periodic table that commonly generate light and from whichsemiconductor light-emitting diodes (LEDs) and semiconductor lasers arefabricated. The various devices of the present invention are able tocouple light out from the surface of a short section of a waveguideformed in a compound semiconductor optical waveguide. The resulting ECSgrating has several advantages including: (1) optimum coupling tosingle- and multi-mode optical fibers; (2) optimum coupling to siliconphotonic waveguides; and (3) economic gains by reducing the real estateused by the grating coupler.

Optical Waveguides with Gratings. FIG. 1 shows a high index contrastsilicon/silicon dioxide photonic waveguide in which the thickness of thecore layer W_(core) may be on the order of 0.1 to 2 microns, the depthW_(g) of the grating may be on the order of 10 to 80% of the totalthickness of the combined core and grating layer, and the free-spacewavelength λ₀ of radiation propagating in the waveguide may be about1550 nm. The core layer may be crystalline silicon (n=3.476 at 1550 nm)with cladding layers of silicon dioxide (n=1.46 at 1550 nm) above andbelow the core. Solely for simplicity, various embodiment of the presentinvention disclosed herein as regards the index of refraction is shownto one decimal place. The skilled artisan will recognize that muchhigher specificity can be used as will be defined by the skilled artisanwhen implementing the present invention. In the example provided of anactual laser (Table 3) and grating (Table 4) structure, for example, theindices of refraction are given to 5 decimal places, which is the outputof the software that calculates the indices.

There are two general types of gratings in semiconductor laserwaveguides. Type 1 is where the layer over the grating has a relativelyhigh index of refraction and would typically include an alloy of a III-Vor II-VI compound. A device that commonly has this grating configurationis a distributed feedback laser. For convenience, this type of gratingis described herein as a “distributed feedback” or DFB grating. Thesecond type of grating found in semiconductor laser waveguides has arelatively low index of refraction over the grating layer. A device thatcommonly has this grating configuration is a distributed Bragg reflector(DBR) laser. For convenience, this type of grating is described hereinas a DBR grating. FIGS. 2 and 3 illustrate a DFB grating and DBR gratingconfiguration.

Practical semiconductor laser structures would have several layers inthe central “core” region and have additional layers for electricalcontacts. Nevertheless, FIGS. 2 and 3 illustrate the key concepts of DFBand DBR lasers and provide a point of comparison for the presentinvention. A DFB laser (FIG. 2) has a high index core region and aslightly lower index for both the substrate layer and the superstratelayer (sometimes referred to as cladding layers). A grating for in-planefeedback (of first, second or higher order) is shown fabricated at aninterface between the core layer and the superstrate layer in FIG. 2.The difference in indices of refraction between the core layer and thesuperstrate layer is modest.

A DBR laser (FIG. 3) also has a high index core region, with a slightlylower index on the substrate side, but on the superstrate side of thecore, the index of the material (which could be air) is low. Inpractice, a thin (˜0.15 micron) layer of silicon dioxide or siliconnitride is often deposited over the grating (which is etched into thecore layers in FIGS. 2 and 3) in a DBR structure.

By observing a formula for the coupling strength k_(pg) between aforward propagating mode (mode p) and backward propagating mode (mode q)of a first-order grating formed in an optical waveguide, [1, 2, 3]:

$\begin{matrix}{\kappa_{pq} = {C_{kn}^{(m)} = {\frac{{\omega ɛ}_{0}}{4}{{b_{m}\left( {n_{1}^{2} - n_{2}^{2}} \right)} \cdot {\int_{- a}^{0}{{E_{p}^{*}(x)}{E_{q}(x)}{dx}}}}}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

where the strength of the grating increases as the difference betweenthe index of refraction of the core region (n₁) and the index ofrefraction of the layer directly above the grating (n₂) increases. Thisterm, (n₁ ²−n₂ ²), is the relative permittivity difference term (sincethe relative permittivity is equal to the square of the index ofrefraction: ε_(rel)=n²). In addition, the effectiveness of the gratingalso increases as the intensity of the light residing in the gratinglayer increases, which is given by the integral term in Equation 1. Thisintegral term is called herein the “grating confinement factor.” Theangular frequency of the radiation is ω, and the permittivity of freespace is ε₀, and b_(m) is the Fourier coefficient corresponding to thefirst-order grating period of the grating profile. Although Equation 1applies to in-plane coupling and not directly to radiation coupled outof or into optical waveguides, increasing the relative permittivitydifference (or index contrast) and grating confinement factor is equallyimportant for gratings of all orders. The importance of high indexcontrast gratings has been recognized in numerous patents [See, e.g.,U.S. Pat. Nos. 7,006,732 B2, 7,068,887], and publications [references 4,5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17].

Another way to view the importance of the relative permittivitydifference and the magnitude of the electric field at the gratinginterface is seen in FIG. 4, which shows that a periodic sinusoidalboundary between two materials is equivalent to a straight boundary witha periodic surface current J(z) given by [18]:

J(z)=−iε ₀ ωηW(ε₁−ε₂)cos(Kz)E(W,z)   Equation 2

where i is the square root of −1, ε₀ is the permittivity of free space,ω is the radian frequency of the electromagnetic (optical) field, ηW isthe amplitude of the periodic boundary, ε₁ is the relative permittivityof the material on the lower side of the grating, ε₂ is the relativepermittivity of the material on the upper side of the grating, K (K=2π/Λwhere Λ is the grating period) is the grating wavevector, z is thespatial coordinate aligned with the axis of the waveguide and is thedirection of propagation of the electromagnetic (optical) mode, andE(W,z) is the value of the electric field at x=W (x is the spatialcoordinate perpendicular to the z axis) for any value of z.

FIGS. 4A and 4B show a periodic boundary (FIG. 4A) is equivalent to astraight boundary (FIG. 4B) with a periodic surface currentJ(z)=−iε₀ωηW(ε₁−ε₂)cos(Kz)E(W,z). In the case of an optical waveguide,we can view the periodic surface current as being generated by a modepropagating in the positive z direction with a longitudinal propagationconstant of β_(p), being strongly phase-matched or coupled to anotherpropagating or radiation mode (with a propagation constant β_(q)) of thewaveguide by the grating vector K[18].

From Equations 1 and 2, we see that the conditions for a strong gratingare: 1) a large relative permittivity difference between the materialson either side of the grating interface; and 2) a large gratingconfinement factor (or a large magnitude of the electric field at thegrating interface). Grating confinement factors can be calculated byapproximating the grating layer with an average relative permittivity[19,20,21].

Such analysis shows that a silicon photonics waveguide has a very largeindex difference term and a very strong grating confinement term. Aconventional DFB waveguide also has a very strong grating confinementterm, but the relative permittivity difference term of a DFB waveguideis small—about an order of magnitude smaller than in a silicon photonicswaveguide. A conventional DBR waveguide has a very large indexdifference term, but has a very small grating confinement factorterm—almost an order of magnitude smaller than a silicon photonicswaveguide. Typical values of the terms related to grating strength for asilicon waveguide, DFB waveguide and DBR waveguide are shown in thefirst three columns of Table 1. The 0.5 number in the first row of Table1 indicates that the period of the grating is approximately equal to thewavelength (measured in the waveguide) of the light propagating in thewaveguide. The second row indicates the thickness of the cover layer forthe ESCG gratings. The third row indicates the difference between(n₁−n₂). The fourth row is the value of the relative permittivitydifference (ε₁−249 ₂) or (n₁ ²−n₂ ²). The fifth row is the gratingconfinement factor as a percentage of the power contained in the gratingregion to the total power in the waveguide. The sixth row is the valueof the intensity of the light propagating in the waveguide at theinterface between the core layer and the bottom of the grating layer.This value corresponds to x=W in FIG. 4. The seventh row in the table isthe product of the two terms discussed above, the relative permittivitydifference and the grating confinement factor that contribute to thegrating strength. This product is defined as an approximate, first-orderFigure of Merit (FOM) of the grating.

TABLE 1 Properties of different types of optical waveguides with gratingcouplers. The results from using the present invention are in the 4right columns labeled ECSG (Enhanced Coupling Strength Grating), ECSG,ECSG and ECSG-Si. Si WG DFB DBR ECSG ECSG ECSG ECSG-SI ∧ (um) — — — 0.50.5 0.5 0.5 Cover layer (nm) — — — 25 10 5 5 Δn_(sub) 2 0.3 0.3 0.3 0.30.3 2 n₁ ²-n₂ ² 10 2.01 10 10 10 10 10 Γ_(grating) 15.3% 15.8%  2.5%11.1% 17.1% 19.3% 30.1% Max ε|_(b) 4.5 2 0.7 1.5 2.1 2.4 3.1 FOM 1.530.32 0.25 1.11 1.71 1.93 3.01

Based on Equation 1, increasing the coupling strength of a gratingcoupler requires both a large value of the relative permittivitydifference (n₁ ²−n₂ ²) and a large value of the grating confinementfactor, which is the case with a silicon/silicon dioxide gratingoutcoupler. A DBR grating has a large value of the relative permittivitydifference but a low value for the grating confinement factor, which isthe opposite of a DFB grating outcoupler.

A way to have a DFB or DBR grating in a compound semiconductor waveguide(such as shown in FIGS. 2 and 3) with both a high index difference and ahigh grating confinement factor is to insert a thin, low index “liner”layer over the high index ridges and grooves of the grating, followed bya thicker high index “cover layer” as shown in FIG. 5. Such aconfiguration is defined herein as an Enhanced Coupling Strength (ECS)grating.

The low index “liner” layer in this ECS grating example provides a largevalue for the (n₁ ²−n₂ ²) term, while the high index “cover” layerresults in a large grating confinement factor by increasing theintensity of the light in the grating layer. The low index “liner” layermaterial could be made from, e.g., silicon dioxide or silicon nitride,and the thickness could be in the range of a few nanometers to many tensor hundreds of nanometers. Typically the thickness of the low indexliner layer will be a small fraction of the wavelength of the radiationpropagating in the waveguide. Low index dielectric materials for a linerlayer include, but are not limited to, SiO, SiO₂, MgF₂, Al₂O₃, HfO₂,Ta₂O₄₋₅, Sc₂O₃, ZrO₂, TiO₂, CaF₂, ThF₄, ZnS, ZnSe, silicon nitride,polymers such as siloxane polymers or others known to one knowledgeablein the art.

The high index cover layer in this ECS grating example need not becrystalline, but could be an amorphous layer of silicon (a-Si), or anamorphous III-V or II-VI layer.

Enhancement of Grating Coupling Strength in High Index Core Waveguides.As discussed above, the simple DFB and DBR gratings shown in FIGS. 2 and3 can have both a high index difference and a high grating confinementfactor if they are modified by inserting a thin, low index liner layerover the high index ridges and grooves of the grating, followed by athicker high index cover layer as shown in FIG. 5. The low index “liner”layer provides a large value for the (n₁ ²−n₂ ²) term, while the highindex “cover layer” results in a large grating confinement factor byincreasing the intensity of the light in the grating layer. The lowindex “liner” layer material could consist of silicon dioxide or siliconnitride and the thickness could be in the range of a few nanometers tomany tens or hundreds of nanometers. Typically the thickness of the lowindex liner layer will be a small fraction of the wavelength of theradiation propagating in the waveguide. The high index cover layer neednot be crystalline, but could be an amorphous layer of silicon (a-Si),or an amorphous III-V or II-VI layer, or any other material with asuitably high index and preferably, low or moderate optical losses. Inaddition, the liner layer or layers may be selected to provide anoptical gain. The index of the cover layer can be somewhat lower, equalor somewhat higher than the index of the core layer, and is taken as 3.5in FIG. 5 just to minimize the number of variables in the example. Ingeneral, for the DFB and DBR cases discussed so far, the index of thecover layer should be chosen to be higher than the “effectiveindex”[1,2,3] of the waveguide, which insures that the solution to thewave equation in the cover layer has an oscillatory solution (and not adamped exponential solution) and will therefore increase the gratingconfinement factor.

For convenience, such a grating structure is described herein to have“liner” and “cover” layers and is called an Enhanced Coupling Strengthgrating, or ECS grating or “enhanced grating”. Columns four (25 nm linerlayer), five (10 nm liner layer) and six (5 nm liner layer) in Table 1show that ECS gratings on the DBR structure in FIG. 5 can have gratingstrengths or crude FOMs that equal or exceed that of high index contrastsilicon/silicon dioxide waveguides with gratings.

Enhancement of Grating Coupling Strength in Low-Index Core Waveguides.The discussion hereinabove has centered on optical waveguides with ahigh index core, corresponding to a core material made of a materialsuch as a semiconductor. However, many optical waveguides, includingboth planar and circular fiber glass and polymer waveguides, have a lowindex core of about 1.5, surrounded by a slightly lower index claddinglayer(s) (or substrate and superstrate layers) of about 1.4 or 1.48.

Equations 1 and 2 provided motivation for using a low-index liner layerbetween two high-index layers at a grating interface (see FIG. 5). Thesame concept applies to a high-index liner layer between two low-indexlayers at a grating interface (see FIG. 6).

FIG. 6 shows a low index core waveguide with a high index (n=3.5) linerand a low index cover layer. For this low index core waveguide, theindex of the liner layer should generally be greater than the effectiveindex of the waveguide structure (to insure an oscillatory solution tothe wave equation in the liner layer), to provide both a large relativepermittivity difference and an increased grating confinement factor.

The improvement in the grating strength of the present invention forsuch a low-index core waveguide is demonstrated by, e.g., the crudeFigure of Merit (FOM) by using a high index liner in FIG. 6 (assummarized in Table 2), in which a lining thickness of 5 nm and 25 nmfor a core thickness of 0.2 microns and a grating depth of 0.1 micronswas analyzed. In this case, the approximate FOM increases withincreasing liner thickness, in contrast to the cases summarized in Table2.

TABLE 2 Properties of a glass optical waveguide (FIG. 6) with a highindex liner layer. Glass WG Glass WG Glass WG (no layer) (5 nm layer)(25 nm layer) Λ (um) — 1 1 Δn_(sub) 0.1 0.1 0.1 |n₁ ²-n₂ ²| 0.29 10 10Γ_(grating) 4.92% 0.66% 21.1% FOM 0.014 0.066 2.11

Enhancement of Grating Coupling Strength in High Index ContrastWaveguides. Although Si/SiO₂ waveguides inherently have strong gratings,the use of a liner layer and cover layer with such waveguides (see FIG.7) can increase their effectiveness. The seventh column in Table 1 showsthat the use of a 5 nm low-index (1.5) liner covered by a 0.15 micronhigh index (3.5) cover layer such as amorphous Si can increase theFigure of Merit of such a waveguide by a factor of 2. In this case, thehigh index cover layer generally should have an index of refractiongreater than the effective index of the waveguide for the same reasonscovered in the discussion of the ECS grating on high index corewaveguides.

Liner and Cover Layer Considerations. To obtain the approximate Figureof Merit terms used in Tables 1 and 2, the inventors solved for thefield distributions of a planar waveguide by assuming average relativepermattivities for regions I, II and III in the grating region shown inFIG. 8. The average relative permattivity of a grating region iscalculated by taking the square root of the weighted (by duty cycle)average relative permittivities for a region [19,20,21]. For the examplein FIG. 8, assuming a liner thickness of 5 nm and a grating period of0.5 microns, the average index index of refraction (which is the squareroot of the average permativity) of region I is 2.693(sqrt(50%*3.5²+50%*1.5²)). Similarly, the average index of region II is3.471 (sqrt(50%*3.5²+2%*1.5²+48%*3.5²)). Finally, the average index ofregion III is 2.655 (sqrt(52%*1.5²+48%*3.5²)).

FIG. 8 is a sketch showing regions I, II and III of a grating with aliner layer. The resulting index profiles and field distributions forliner thicknesses of 5, 10 and 25 nm are shown in FIGS. 9A and 9B, usinga constant total grating depth of 0.1 microns, a waveguide corethickness of 0.2 microns, and a high-index cover layer of 0.15 microns.

FIGS. 9A and 9B are graphs of index profiles and field intensity plots.FIG. 9A shows the index profile for the waveguide structure shown inFIG. 5 for a liner thickness of 5, 10 and 25 nm with the grating layerreplaced by the average relative permittivities in grating regions I, IIand III. FIG. 9B shows the resulting intensity profiles for thefundamental mode for the index profiles in FIG. 9A. The basic waveguideindex profile shown in FIG. 9A can be used to vary the thicknesses andindices of the layers of the structure to estimate initial optimizationsof the approximate Figure of Merit for an ECS grating. Examples of suchlayer variation plots are shown in FIGS. 10 through 14.

FIGS. 10A and 10B are graphs that show the index profile and gratingconfinement as a function of the thickness of the cover layer. In thisnon-limiting example, the grating period was 0.2 um and the linerthickness 10 nm. FIG. 10A shows the index profile of the ECS gratingwaveguide (shown in FIG. 5) with a fixed core thickness of 0.2 microns,a grating thickness of 0.3 microns, a liner thickness of 10 nm and avariable thickness for the high index cover layer. FIG. 10B shows thegrating confinement factor as a function of the thickness of the highindex cover layer. In FIGS. 10A and 10B, all of the thicknesses of thelayers are fixed except the high index cover layer. For this particularchoice of layer thicknesses, indices and grating period, the optimumthickness for the cover layer is about 0.3 microns.

FIGS. 11A to 11C show the index profile, grating confinement factor andfield intensity plots as a function of core thickness. In thisnon-limiting example, the grating period was 0.2 um and the linerthickness 10 nm. In FIG. 11A an index profile of the ECS gratingwaveguide (FIG. 5) with a variable core thickness, a grating thicknessof 0.2 microns, and a thickness for the high index cover layer of 0.2microns is shown. FIG. 11B shows the grating confinement factor as afunction of the thickness of the core thickness. FIG. 11C shows a fieldintensity plots for core thicknesses ranging from 0.1 to 0.5 microns. InFIG. 11, all of the thicknesses of the layers are fixed except that ofthe core layer. For this particular choice of fixed parameters, theoptimum thickness of the core layer is just under 0.1 micron.

FIGS. 12A to 12C show the index profile, grating confinement factor andfield intensity plots as a function of the thickness of the gratinglayer. In this non-limiting example, the grating period was 0.2 um andthe liner thickness 10 nm. FIG. 12A shows the index profile of the ECSgrating waveguide (FIG. 5) with a fixed core thickness of 0.2 microns, avariable grating thickness and a 0.1 micron thickness for the high indexcover layer. FIG. 12B shows the grating confinement factor as a functionof the thickness of the grating layer. FIG. 12C shows the fieldintensity plots for grating thicknesses ranging from 0.1 to 0.5 microns.In FIGS. 12A to 12C, all of the thicknesses of the layers are fixedexcept the thickness of the grating layer. For this particular choice oflayer thicknesses, indices and grating period, the grating confinementfactor approaches 0.5 for a grating thickness of about 0.5 microns. Theresulting field intensities are plotted as a function of grating depth.

FIG. 13A shows a sketch of a grating with liner layer and cover layers.In this non-limiting example, the grating period was 0.5 um and theliner thickness 25 nm. FIG. 13B shows the grating confinement factor asa function of grating depth for core thicknesses varying from 0.1 to 0.4microns. FIG. 13C shows the field intensity plots for a fixed gratingdepth of 0.1 microns for core thicknesses varying from 0.1 to 0.4microns.

FIG. 14A shows a sketch of the structure and FIG. 14B shows a plot ofthe magnitude of the field intensities at the lower boundary of thegrating for a normalized core thickness of W/λ, for index differencesbetween the core and the substrate ranging from 0.1 to 0.5. In thisnon-limiting example, the grating period was 0.5 um and the linerthickness 5 nm.

The discussion hereinabove serves only to show general trends anddependences of the grating confinement factor and the FOM of variousoptical waveguides with ECS gratings, and is not an attempt to optimizeany particular ECS grating waveguide. The above analysis is only afirst-order analysis and approximates the grating by several layers,each with an average relative permittivity. There are also additionalconstraints on the configuration of optical waveguides with ECSgratings. For example, the minimum thickness of the core of a high indexcontrast silicon waveguide may be constrained by electronic devices thatare fabricated in other regions of the die. Or a III-V waveguide designmay be constrained by a laser structure or other photonic devices thatshare common waveguide layers with the ECS grating waveguide region. Assuch, the skilled artisan will recognize that using the designparameters provided herein, there are various designs that could providean “optimum” ECS waveguide design for a specific application.

A comparison of the field intensities for the waveguides shown in FIGS.1, 2, 3, 5 and 7 is shown in FIG. 15 for a waveguide thickness of 0.2microns, a grating period of 0.5 um, and a grating depth of 0.1 microns.The first intensity profile on the left is for the silicon/silicondioxide waveguide of FIG. 1; the second intensity profile is for the DFBwaveguide of FIG. 2 with core and cladding indices of 3.5 and 3.2; thethird intensity profile is for the DBR waveguide of FIG. 3 with a coreindex of 3.5, a substrate index of 3.2 and a superstrate index of 1.5;the fourth intensity profile is for the ECS grating waveguide of FIG. 5with a core index of 3.5, a substrate index of 3.2, a 5 nm SiO₂ liner,and an amorphous silicon layer of 0.15 microns; and the fifth intensityprofile is of an ECS Si/SiO₂ grating waveguide shown in FIG. 7 with a 5nm SiO₂ liner, and an amorphous silicon layer of 0.15 microns. Thegrating confinement factor is the area under the intensity profile inthe grating region. The enhancement of the low grating confinementfactor of the DBR waveguide (FIG. 3) is apparent when compared to theECS waveguide (FIG. 5), which is the DBR waveguide with the addition ofa 5 nm silicon dioxide liner layer and a 0.15 micron amorphous siliconlayer. Although the DFB waveguide (FIG. 2) has a large confinementfactor, about the same as the silicon/silicon dioxide waveguide (FIG.1), the small relative permittivity difference between the materials oneither side of the grating limits the strength of the DFB grating.Comparing the fifth intensity profile with the first intensity profile,we see that the addition of the liner and a cover layer increases thegrating confinement factor and also shifts the peak of the propagatingmode from the core layer to the grating layer. By varying the thicknessof either or both of the liner layer and/or cover layer, the overlapbetween the intensity distribution in the smooth (no grating) region ofthe waveguide and the grating region of the waveguide can be increasedwhile simultaneously increasing the efficiency of the grating.

Calculation of Grating Coupling Length. All of the figures above areanalyzed using a simple planar layer waveguide analysis that does notaccount for the interaction of the waveguide mode with the exact gratingboundaries. As a result, the calculated Figure of Merit (FOM) is anapproximate number that is mainly useful for initial exploration of theparameter space of an optical waveguide with an ECS grating, which canbe optimized for specific applications as taught herein. A more completeanalysis of such structures, such as a numerically exact Floquet-Blochapproach [19,20], finite element [22], boundary element [23] or finitedifference time domain [24] approach that matches the electromagneticfield at every interface contained in the grating region (and at everyinterface in the optical waveguide) is required to obtain a detailed andaccurate solution of the grating strength and distance over which alarge fraction of the waveguide light is radiated by the grating. In theFloquet-Bloch approach, the electric field is written as:

$\begin{matrix}{{E_{y}\left( {x,z} \right)} = {\sum\limits_{n = {- \infty}}^{\infty}{{f_{n}(x)} \cdot {\exp \left( {{- {jk}_{zn}}z} \right)}}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

where f_(n)(x) is the transverse variation of each space harmonic andthe longitudinal propagation constant k_(zn) of each space harmonic isgiven by:

$\begin{matrix}{{k_{zn} = {{\beta_{m} + {j\; \alpha}} = {\left( {\beta_{0} + {nK}} \right) + {j\; \alpha}}}},{\alpha < 0},{K = \frac{2\pi}{\Lambda}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

where β₀, is equal to 2π/λ_(g) (λ_(g) is the wavelength of the fieldvariation along the z axis of the waveguide) and α is the electric fieldattenuation constant (2α is the power attenuation constant), which isproportional to the amount of light radiated from the waveguide.

Since the power attenuation is proportional to exp(−2αz), the intensityof the light in the waveguide will be reduced to 1/e in a distancez=L=NΛ=1/(2α), where N is the number of grating periods. Therefore thelight intensity in the waveguide is reduced to 1/e in N=1/(2αΛ) gratingperiods. A Floquet-Bloch approach was used to find the attenuationcoefficient for the structure shown in FIG. 5 as a function of linerlayer thickness assuming a core thickness of 0.2 microns, a gratingthickness of 0.1 microns, a grating period of ˜0.4765 microns, a highindex cover layer thickness of 0.15 microns and a free space wavelengthof 1550 nm. FIG. 16 is a plot of the normalized attenuation constant(the product of the peak attenuation coefficient and the second-ordergrating period) as function of liner layer thickness and indicates thatthe optimum choice for the liner layer thickness is about 20 nm for thespecific structure in FIG. 5. For this case, the light remaining in thewaveguide is reduced by 1/e in N ˜22 grating periods or about 11 micronsfor αΛ=0.023.

For other waveguides with ECS grating couplers, the optimum liner layerthickness will vary. The lower limit on liner layer thickness will bedictated by deposition, fabrication and processing considerations suchas the minimum thickness required to insure complete coverage of theliner layer over the grating or over desired portions of the grating.

All of the waveguide calculations described herein assume a free spacewavelength of 1550 nm, however the skilled artisan will know how to varythis space. The field plots apply to other free space wavelengths λ₀ ifthe dimensions of the layers making up the waveguides are multiplied byλ₀/1550 (for λ₀ in units of nm). The thickness of the liner layer in anECS grating waveguide will generally be a small fraction of the freespace wavelength in the case of a continuous low index liner (FIG. 23)between a high index core and a high index cover layer. The reason forthis, but in no way a limitation of the present invention, is that thefield distribution in the (low index) liner layer has an exponentiallydecaying profile, so the liner has to be thin enough that there is asubstantial value of the electric field at the liner/cover interface. Asan example, the optimum liner thickness of 20 nm for the example basedon FIG. 5 and described above is only 1.3% of the free space wavelength.

However, the waveguide in FIG. 6 has a low index core, a high indexliner and a low index cover layer. In this case the field distributionin the liner layer has an oscillatory solution and the crude Figure ofMerit increases with increasing liner thickness (Table 2). In this case,increasing the liner thickness sufficiently can shift the peak andnarrow the field distribution.

For simplicity, the analysis in all of the above sections assumes ECSgratings with a constant 50% duty cycle, constant period and a uniformdepth. The intensity profiles along such gratings have an exponentiallydecaying profile. To obtain other intensity profiles along a gratingcoupler, such as a Gaussian, the duty cycle, grating depth and/orgrating period may be varied. The application of liner and cover layersto such gratings to enhance their coupling strength are covered by theclaims in this patent. The grating profiles used in all of theillustrations are rectangular, although the application of liner andcover layers can be applied to any grating profile to enhance thecoupling strength and are covered by the claims of this patent.

Reduction of Optical Losses at Waveguide Discontinuities. Anotherconcern with gratings in waveguides is the interface between thewaveguide region without a grating and the waveguide region with agrating. For example, in the manufacture of DBR lasers it is common toetch away the top contact (cap) layer and a significant portion of thep-clad layer before the grating is formed [25].

FIG. 17 is a sketch that shows the layers in the laser region(Section 1) and in intermediate regions (Section 2 and 3), and the ECSgrating region (Section 4) for the laser and grating layers shown inTables 3 and 4. FIG. 17 shows stepped cross sections of the transitionregions from a laser waveguide section (Table 3) to the ECS gratingoutcoupler region formed in the laser structure (Table 4). Since theindex profiles differ in the laser, transition and grating regions,there are radiation losses (and reflections) at each interface. Table 3lists epitaxial layer thicknesses and indices (assuming an emissionwavelength of 1550 nm) for the laser structure shown in FIG. 17, andTable 4 lists layer thicknesses and indices for an ECS grating waveguideformed from the laser structure.

In this example, a 0.2 micron thick grating is etched into the InGaAsPseparate confinement heterostructure (SCH) layer (FIG. 17). Etchingother InGaAsP/InP laser structures may result in a grating etched intoboth an InP clad layer and an InGaAsP Separate ConfinementHeterostructure layer which requires adding a fourth “average relativepermittivity” layer as illustrated in FIG. 18B. FIGS. 18B and 18D showsthat a grating ridge can contain layers with multiple indices if etchingof the grating continues through the superstrate and into the corelayer.

A good approximation of the radiation loss and reflectivity at a stepdiscontinuity in an optical waveguide is given by 1−κ_(x), where κ_(x)is an overlap integral κ_(x) [21]:

κ_(x)|∫_(−∞) ^(∞) E _(g)(x)E _(w)*(x)dx| ²/(∫_(−∞) ^(∞) E _(g)(x)E_(g)*(x)dx∫ _(−∞) ^(∞) E _(w)(x)E _(w)*(x)dx),   Equation 5

κ_(x) is the normalized intensity overlap integral of the fields oneither side of the discontinuity, E_(w) is the electric fielddistribution on one side of the discontinuity and E_(g) is the fieldintensity distribution on the other side of the discontinuity.

FIGS. 18A to 18D show various details of the gratings after etching anddeposition of the liner layer and cover layers. In FIG. 18A, for thestructure shown in FIG. 15, the grating ridges contain InP(index=3.16492), the layer below the grating is InGaAsP (index=3.35110),the liner layer is silicon dioxide (index=1.46) and the cover layer isamorphous silicon (index=3.476). In FIG. 18B, etching a grating throughboth an InP layer and an InGaAsP layer requires adding a fourth “averagerelative permittivity” layer. In FIG. 18C, for the glass waveguidestructure shown in FIG. 6, the grating ridges contain only material withan index of refraction=1.4. In FIG. 18D, for the same structure of FIG.6, the grating ridges or teeth could contain material with an index ofrefraction of 1.4 and 1.5 if the grating is etched past the claddinglayer into the core layer.

FIGS. 19A to 19C show index profiles and field intensities for thecomplete laser structure and for three other sections of the combinationlaser-ECS grating structure. FIG. 19A shows plots of the fieldintensities and index profiles in the laser waveguide region (section 1of FIG. 17, square index profile and square field plot); in section 2 ofFIG. 17 with no liner or cover layer (“x” index profile and “x” fieldplot, 93% overlap); and in section 2 of FIG. 17 with a liner layer andan amorphous Si cover layer (triangle index profile and triangle fieldplot, 99% overlap). FIG. 19B shows plots of the field intensities andindex profiles in the laser waveguide region (section 1 of FIG. 17,square index profile and square field plot); in section 3 of FIG. 17with no liner or cover layer (“x” index profile and “x” field plot, 73%overlap); and in section 3 of FIG. 17 with only an amorphous Si coverlayer (triangle index profile and triangle field plot, 93% overlap); andin section 3 of FIG. 17 with a liner and amorphous Si cover layer(circle index and field plot, 97% overlap). FIG. 19C shows plots of thefield intensities and index profiles in the laser waveguide region(section 1 of FIG. 17, square index profile and square field plot); andin section 4 of FIG. 17 with a liner and cover layer (“x” index profileand “x” field plot, 94% overlap).

This example shows how the application of a liner and cover layer candecrease radiation losses at waveguide discontinuities.

TABLE 3 Epitaxial layers of the 5 QW laser Structure Thickness MaterialLayer Composition (um) Index Air — 1 Cap InGaAs 0.100 3.62525 P-clad InP2 3.16492 SCH In_(0.74)Ga_(0.26)As_(0.5)P_(0.5) 0.244** 3.35110 4xbarriers In_(0.74)Ga_(0.26)As_(0.5)P_(0.5) 0.008 3.35110 5x QWsIn_(0.74)Ga_(0.26)As_(0.81)P_(0.19) 0.006 3.50636 SCHIn_(0.74)Ga_(0.26)As_(0.5)P_(0.5) 0.044* 3.35110 N substrate InP —3.16492 *Sum of the thickness of the SCH, barriers, and QWs is 0.15micron. **The grating with 0.2 micron etched depth will be in InGaAsPSCH layer.

TABLE 4 Layers in the Grating Section of the 5 QW Laser StructureThickness Material Layer Composition (um) Index Air — 1 A-Si AmorphousSilicon 0.230 3.476 Grating 3 60% SiO₂ + 40% Si 0.025 2.47224* Grating 250% InGaAsP + 10% SiO₂ + 40% Si 0.175** 3.26514* Grating 1 50% InGaAsP +50% SiO₂ 0.025 2.58471* SCH In _(0.74)Ga_(0.26)As_(0.5)P_(0.5) 0.044***3.35110 4x barriers In_(0.74)Ga_(0.26)As_(0.5)P_(0.5) 0.008 3.35110 5xQWs In_(0.74)Ga_(0.26)As_(0.81)P_(0.19) 0.006 3.50636 SCH In_(0.74)Ga_(0.26)As _(0.5)P_(0.5) 0.044*** 3.35110 N InP — 3.16492 substrate*Grating indicates calculation referenced in FIG. 19. **Total thicknessof grating 1-3 is 0.225 micron. ***Sum of the total thickness of theSCH, barriers, and QWs is 0.15 micron.

FIG. 20 shows the calculated normalized complex propagation constants ofthe ECS grating coupler shown in FIG. 19C. The plot in FIG. 20 shows aplot of normalized reciprocal wavelength (Λ/λ₀) as a function ofnormalized real part of the longitudinal propagation constant (β/K) andnormalized imaginary part of the longitudinal propagation constant (orattenuation) (αΛ) calculated using the Floquet-Bloch approach for theECS grating coupler shown in FIG. 19C and Table 4. For this structure,the light remaining in the grating waveguide is reduced by 1/e in N ˜13grating periods or about 6.5 microns (since αΛ˜0.08) for a wavelength of1550 nm and a grating period of 0.4882 microns.

FIG. 21 shows the fraction of incident power radiated down (⋄), fractionof incident power radiated up (*), fraction of incident power reflectedbackwards (•), fraction of incident power transmitted forward (▪);fraction of incident power radiated both upwards and downwards (▴) andthe sum of the fractions of incident power reflected, transmitted andradiated (♦) as a function of wavelength (assuming a fixed grating witha 50% duty cycle) for the waveguide shown in FIG. 19C and Table 4. Thedeviation of the total power plot (♦) from unity indicates that themaximum error of the Floquet Bloch analysis is about 10% at a wavelengthof 1550 nm and drops to less than 3% for wavelengths greater than 1560nm. The points (1), (2), (3) and (4) in FIG. 21 correspond to the samefour points shown in FIG. 20. These calculations assumed a gratinglength of 19.5 microns and a grating period of 0.4882 microns.

FIGS. 22A to 22D show: FIG. 22A shows the intensity distribution in the5 QW grating waveguide region shown in FIG. 19C and Table 4 at point (1)in FIGS. 20 and 21, which is the peak of the attenuation curve andoccurs at a wavelength of 1565 nm. At point (1) the total radiated poweris ˜20% and the total reflected power is ˜80%, which illustrates thatthe wavelength of maximum attenuation is not the desired wavelength formaximum outcoupled radiation. FIG. 22B shows the intensity distributionin the 5 QW grating waveguide region shown in FIG. 19C and Table 4 atpoint (2) in FIGS. 20 and 21, which corresponds to the wavelength (1592nm) at which the maximum total power is radiated (74%) and the reflectedpower is ˜10%. FIG. 22C shows the intensity distribution in the 5 QWgrating waveguide region shown in FIG. 19C and Table 4 at point (3) inFIGS. 20 and 21, which corresponds to the wavelength (1610 nm) at whichthe total power radiated is ˜67% and the power reflected is low (˜5%).FIG. 22D shows the intensity distribution in the 5 QW grating waveguideregion shown in FIG. 19C and Table 4 at point (4) in FIGS. 20 and 21,which corresponds to the wavelength (1630 nm) at which the total powerradiated is ˜60% and the power reflected is very low (less than 1%). Thewhite lines within the frame of the colored plots outline the ridges andgrooves of the grating. For wavelengths sufficiently far away from aBragg resonance, the intensity distribution within a grating cycleremains constant along the direction of propagation.

FIGS. 23A to 23H shows a simple ECS grating waveguide 10 that includes asubstrate 12 onto which a core 14 is grown or deposited that includesfeatures or gratings 19 consisting of ridges 16 separated by grooves 17with a continuous liner layer 18, shown in this embodiment to cover theridges 16 and grooves 17, which together form a grating 19. The skilledartisan will recognize that FIGS. 23 to 30 are side views of thegratings structure in which the one or more gratings 19 appear as ridges16, with grooves 17 therebetween. Disposed on the grating 10, is theliner layer 18 that is deposited or formed on the grating 19, Finally, acover layer 20, is deposited on the liner layer 18. In certain examples,the gratings 19 can be designed to have a period that is equal to aboutthe wavelength of the light propagating in the optical waveguide toproduce an outcoupling in about 10 to 50 grating cycles. In this case,the grating is referred to as a second-order grating.

In another example, the gratings 19 can be defined as having a periodthat is equal to about half the wavelength of the light propagating inthe optical waveguide, to produce in-plane reflection withoutoutcoupling. The period and order of the grating Λ is defined further bythe Bragg condition as Λ=n λ_(m)/2 where n is an integer correspondingto the Bragg order, λ_(m) is the wavelength of light in the waveguide(λ_(m)=λ₀/n_(eff)) and n_(eff) is the effective index of the modepropagating in the waveguide. For a typical III-V waveguide at a freespace wavelength of about 1550 nm, the effective index is about 3.3 andthe first order grating period is close to ¼ micron and the second ordergrating period is close to ½ micron. The concept of this inventionapplies to waveguide gratings of any Bragg order. FIGS. 23A to 23H showexamples of different, non-limiting, grating profiles that can be usedwith the present invention.

FIG. 24 shows an alternative embodiment of the ECS grating waveguide 10with a segmented cover layer only in the bottom of the groove 17 and thetops of the ridge 16, which are on a core 14 and substrate 12. As such,the liner layer 18 does not have to be continuous and can take manyforms, as shown in FIGS. 24 through 30. Briefly, FIG. 24 shows thesubstrate 12, core 14 and ridges 16 separated by grooves 17. The coverlayer 20 is deposited (after the liner layer is formed) on the exposedsurfaces of the grating 19 and liner layer 18. The liner layer 18 can bemade with a variable thickness to provide at least one of the following:variable strength of the coupling, an effective variable duty cycle, aneffective variable grating depth, a Gaussian profile of the radiatedfield profile along the outcoupler grating, or a near-Gaussian profileof the radiated field profile along the outcoupler grating. In thisembodiment, a liner layer 18 is not continuous and is shown as a topliner 26, which is on top of the ridges 16 and bottom liner 28, which isin the bottom of the groove 17.

FIG. 25 shows another embodiment of the ECS grating waveguide 10 withthe core 14 and substrate 12, and in which a segmented liner layerformed only on the ridge 16 sidewall 22 and ridge sidewall layer 24. Acover layer 20 is deposited after the liner layer is formed.

FIG. 26A to 26C show three embodiments of the ECS grating waveguide 10,which include the core 14 and substrate 12. In FIG. 26A the grooves 17between ridges 16 are filled with liner material that forms the linerlayer 18 in the grooves 17. A cover layer 20 is deposited after theliner layer is formed. In FIG. 26B, the grooves 17 between the ridges 16are partially filled with a liner material that forms the liner layer18. A cover layer 20 is deposited after the liner layer is formed. InFIG. 26C, the grooves 17 between the ridges 16 are over filled with aliner material that extends above the ridges and forms the liner layer18. The cover layer 20 is deposited after the liner layer is formed.

FIG. 27 shows another embodiment of the ECS grating waveguide 10 withhorizontal multi-layers of multiple liner layers 18 and multiple coverlayers layer 20, which are formed vertically within and outside thegrooves 17 and over the ridges 16 (of course, they could also end at thetop of the ridges 16). Not shown is a similar ECS grating with verticalmulti-layers of liners and cover layers.

FIG. 28 shows another embodiment of the ECS grating waveguide 10 withmulti-layers of liner layers 18 and cover layers 20 on all surfaces ofthe ridges 16 and grooves 17, wherein each layer of the liner layer 18and cover layer 20 are deposited in an alternating manner and that coator follow the contour of the ridges 16 and the grooves 17.

FIG. 29 shows another embodiment of the ECS grating waveguide 10, whichincludes a core 14 on a substrate 12, and in this embodiment includes anasymmetric liner layer 18 that covers the bottom of the groove 17, asidewall of the ridge 22 and the top of the ridge 16. After the linerlayer 18 is formed, the cover layer 20 is deposited.

FIG. 30 shows an alternate configuration of an asymmetric liner layerECS grating waveguide 10, which includes the core 14 on a substrate 12,and in this embodiment includes an asymmetric liner layer 18 that coversa sidewall layer 22 and the top of the ridge 16. After the liner layeris formed, the cover layer 20 is deposited. The configurations of FIGS.29 and 30 can be used to form an effective blazed grating.

FIG. 31A shows an isometric view of an ECS grating waveguide device 40on a simple waveguide such as shown in FIGS. 5 and 6. FIGS. 31A to 31Falso shows a mesa-waveguide structure to provide lateral confinement. Inthis figure, light is emitted from the device 10 that is formed on asubstrate 12 and core 14 below the mesa waveguide 46. The followingfigures show one non-limiting example of how an ECS grating 10 that ispart of a waveguide 40 may be fabricated in accordance with the presentinvention and can also be understood with, e.g., FIGS. 23 to 30, whichshow various non-limiting embodiments of the present invention, whichthe skilled artisan will be able to construct using standard deposition,etching, photolithographic and other techniques as disclosed herein.FIG. 31B is a top view of an ECS grating coupler integrated with a DBRlaser and shows how a narrow (0.1 to 5 microns) mesa-waveguide can beexpanded in a very short distance to produce an ECS grating withdimensions of about 10 microns by 10 microns for efficient coupling tofiber optics and/or other grating waveguide couplers. FIG. 31C is a sideview of the integrated ECS grating coupler of FIG. 31B. FIG. 31D showsan alternative form of the waveguide mesa in which the gratings arepositioned on the top of the mesa, FIG. 31E shows the waveguide on thetop surface just outside the mesa, and 31F shows the waveguidepositioned along the sides of the mesa. FIG. 31G shows that the gratingsthat can occur on both top surfaces or on both top surfaces and thesides of the mesa.

Most of the figures herein have been shown with a one-dimensionalcross-section in the x direction (x direction being defined asperpendicular to the direction of light propagating in the waveguide inthe z direction). However, it is common to etch a continuous mesaaligned along the z direction. Once a mesa is etched to constrain thelight in the y direction in addition to the x direction, then there isthe choice of: (1) forming the grating over both the top of the mesa andthe surface outside the mesa (FIG. 31B); (2) placing the grating just onthe top of the mesa (FIG. 31D, or mainly on top of the mesa—there may beremnants of a grating outside the mesa due to fab methods); (3) placingthe grating just outside the mesa (FIG. 31E); and (4) wherein the sidesof the mesa can be a grating (FIG. 31F).

FIG. 32 shows the initial step in fabricating an ECS grating coupler 40requires covering the substrate or wafer 12 with layers 14 and 42 usedto make the waveguide with photoresist 44 (also shown in this embodimentwith a core 14), and a superstrate 42. The left, middle and right viewcorresponds to the end, side and top view of a section of the wafer 12.

FIG. 33 shows a second step in fabricating an ECS grating coupler 40that shows the definition in photoresist 44 of a mesa waveguide 46 inwhich the photoresist 44 is exposed and developed to form a pattern onthe superstrate 42. The left, middle and right view corresponds to theend, side and top view of a section of the substrate or wafer 12.

FIG. 34 shows a third step in fabricating an ECS grating coupler 40 thatdefines the mesa waveguide 46 by etching away a section of thesuperstrate 42. The left, middle and right view corresponds to the end,side and top view of a section of the substrate or wafer 12. The etchcan be, e.g., a wet etch, a plasma etch, an ion beam etch (IBE), areactive ion etch (RIE), an induction coupled plasma (ICP) etc., orequivalent etch.

FIG. 35 shows a fourth step in fabricating an ECS grating coupler 40that protects the mesa waveguide 46 with photoresist 44 and defines agrating in photoresist 48. The photoresist grating 48 can be formed byseveral procedures such as, e.g., holography, e-beam writing, orstandard lithography. The left, middle and right view corresponds to theend, side and top view of a section of the wafer.

FIG. 36 shows a fifth step in fabricating an ECS grating coupler 40 isreplicating the photoresist grating 48 into the waveguide by any ofseveral etching procedures such as, e.g., wet etching, plasma etching,ion beam etching, reactive ion etching, chemically assisted ion beametching, or inductively coupled plasma etching. The photoresist 48 willbe removed prior to the next processing step. The left, middle and rightview corresponds to the end, side and top view of a section of thewafer.

FIG. 37 shows a sixth step in fabricating an ECS grating coupler 40, inwhich a liner layer 50 (made with a material such as silicon dioxide orsilicon nitride) is deposited over a high index superstrate material 42that was etched in the previous step. The left, middle and right viewcorresponds to the end, side and top view of a section of the wafer.(Note: If the superstrate material consisted of a low index material,the liner layer would be made from a high index material such asamorphous Si.)

FIG. 38 shows a final step in fabricating an ECS grating coupler 40 inwhich a high index cover layer 52 (such as amorphous silicon, or a III-Vmaterial, such as GaAs, AlGaAs, GaP, InP,

InGaAsP, AlGaInAs, or AlGaInAsP) is deposited over the low index linerlayer 50. The left, middle and right view corresponds to the end, sideand top view of a section of the wafer. The wavelength of light can bevaried from visible, ultraviolet, and/or infrared depending on thematerials used to make the device.

The discussion so far in this patent has concentrated on gratings usedfor coupling radiation into or out of an optical waveguide or ongratings used for in-plane reflection for feedback. However thisinvention applies to all types of gratings, such as multiple periodicgratings [26], distributed Bragg reflectors [27], and grating assisteddirectional couplers [20].

It will be understood that particular embodiments described herein areshown by way of illustration and not as limitations of the invention.The principal features of this invention can be employed in variousembodiments without departing from the scope of the invention. Thoseskilled in the art will recognize, or be able to ascertain using no morethan routine experimentation, numerous equivalents to the specificprocedures described herein. Such equivalents are considered to bewithin the scope of this invention and are covered by the claims.

All publications and patent applications mentioned in the specificationare indicative of the level of skill of those skilled in the art towhich this invention pertains. All publications and patent applicationsare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference.

The use of the word “a” or “an” when used in conjunction with the term“comprising” in the claims and/or the specification may mean “one,” butit is also consistent with the meaning of “one or more,” “at least one,”and “one or more than one.” The use of the term “or” in the claims isused to mean “and/or” unless explicitly indicated to refer toalternatives only or the alternatives are mutually exclusive, althoughthe disclosure supports a definition that refers to only alternativesand “and/or.” Throughout this application, the term “about” is used toindicate that a value includes the inherent variation of error for thedevice, the method being employed to determine the value, or thevariation that exists among the study subjects.

As used in this specification and claim(s), the words “comprising” (andany form of comprising, such as “comprise” and “comprises”), “having”(and any form of having, such as “have” and “has”), “including” (and anyform of including, such as “includes” and “include”) or “containing”(and any form of containing, such as “contains” and “contain”) areinclusive or open-ended and do not exclude additional, unrecitedelements or method steps.

The term “or combinations thereof” as used herein refers to allpermutations and combinations of the listed items preceding the term.For example, “A, B, C, or combinations thereof” is intended to includeat least one of: A, B, C, AB, AC, BC, or ABC, and if order is importantin a particular context, also BA, CA, CB, CBA, BCA, ACB, BAC, or CAB.Continuing with this example, expressly included are combinations thatcontain repeats of one or more item or term, such as BB, AAA, AB, BBC,AAABCCCC, CBBAAA, CABABB, and so forth. The skilled artisan willunderstand that typically there is no limit on the number of items orterms in any combination, unless otherwise apparent from the context. Incertain embodiments, the present invention may also include methods andcompositions in which the transition phrase “consisting essentially of”or “consisting of” may also be used.

As used herein, words of approximation such as, without limitation,“about”, “substantial” or “substantially” refers to a condition thatwhen so modified is understood to not necessarily be absolute or perfectbut would be considered close enough to those of ordinary skill in theart to warrant designating the condition as being present. The extent towhich the description may vary will depend on how great a change can beinstituted and still have one of ordinary skilled in the art recognizethe modified feature as still having the required characteristics andcapabilities of the unmodified feature. In general, but subject to thepreceding discussion, a numerical value herein that is modified by aword of approximation such as “about” may vary from the stated value byat least ±1, 2, 3, 4, 5, 6, 7, 10, 12 or 15%.

All of the compositions and/or methods disclosed and claimed herein canbe made and executed without undue experimentation in light of thepresent disclosure. While the compositions and methods of this inventionhave been described in terms of preferred embodiments, it will beapparent to those of skill in the art that variations may be applied tothe compositions and/or methods and in the steps or in the sequence ofsteps of the method described herein without departing from the concept,spirit and scope of the invention. All such similar substitutes andmodifications apparent to those skilled in the art are deemed to bewithin the spirit, scope and concept of the invention as defined by theappended claims.

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What is claimed is:
 1. A grating, comprising: a substrate; a corecomprising a first material having an index of refraction (n_(core))disposed on the substrate; one or more ridges and one or more groovescomprising the first material formed on the core, wherein the one ormore grooves are adjacent to, or between the one or more ridges, and theridges and grooves form a grating; a liner layer comprising a secondmaterial having an index of refraction (n_(liner)) disposed on at leasta portion of a grating cycle; an amorphous or crystalline cover layercomprising a third material having an index of refraction (n_(cover))disposed on the liner layer; a superstrate or air layer disposed abovethe cover layer; and wherein the third material is not the same as thefirst material, n_(core)≠n_(liner) and n_(liner)≠n_(cover).
 2. Thegrating of claim 1, wherein the liner layer is disposed on at least oneof: the bottom of the groove; one or more sidewalls of the ridges; onthe top of the ridges; two or more liner layers in the groove; or on thesides of the ridges that do not have a top.
 3. The grating of claim 1,wherein the liner layer selected from one or more of the followingoptional configurations: (a) the liner layer is not contiguous; (b) theliner layer is disposed on a first sidewall, a second sidewall or boththe first and second sidewalls of the ridges; (c) the liner layer isdefined further as one or more liner layers that are contiguous and thatfollow the contour of the ridges and the grooves; (d) the liner layer isnot contiguous, wherein the liner layer is defined further as beingsubstantially parallel to a bottom of the one or more grooves, and thenon-contiguous layers are separated by one or more amorphous orcrystalline cover layers; (e) the liner layer is defined further as twoor more liner layers that are contiguous and that follow the contour ofthe ridges, and each of the two or more liner layers are separated byone or more amorphous or crystalline cover layers; (f) the liner layeris disposed on one or more tops of the ridges, one or more groovesbetween the ridges, or both the top of the ridges, and the groovesbetween the ridges; (g) the liner layer is disposed on a first sidewall,one or more tops of the ridges, and one or more grooves between theridges, to provide an effective blazed grating; (h) the liner layer isdisposed on a first sidewall and one or more tops of the ridges; or (i)the liner layer is disposed on one or more first sidewall or secondsidewall of one or more waveguiding structures for grating coupling(inward or outward).
 4. The grating of claim 1, wherein the liner layeris disposed on a high index contrast Si/SiO₂ waveguide to furtherenhance the performance of the grating.
 5. The grating of claim 1,wherein the amorphous or crystalline cover layer is selected to providethe second difference in the index of refraction between the amorphousor crystalline cover layer and the liner layer that is similar to thefirst difference in the index of refraction provided between the coreand the liner layer.
 6. The grating of claim 1, wherein the liner layeris selected from at least one of SiO, SiO₂, MgF₂, Al₂O₃, HfO₂, Ta₂O₄₋₅,Sc₂O₃, ZrO₂, TiO₂, CaF₂, ThF₄, ZnS, ZnSe, polymers, and silicon nitride.7. The grating of claim 1, wherein the liner layer comprises a variablethickness to provide at least one of varying the strength of thecoupling, an effective variable duty cycle, an effective variablegrating depth, a Gaussian profile in a radiating coupler grating, or anear-Gaussian profile in a radiating coupler grating.
 8. The grating ofclaim 1, wherein the liner layer is selected to provide at least one ofan optical loss or an optical gain.
 9. The grating of claim 1, whereinthe optical waveguide is at least one of a distributed Bragg reflectors(DBRs) or a distributed Bragg deflectors (DBDs).
 10. The grating ofclaim 1, wherein the optical waveguide is defined further as comprisingat least two ECS gratings to make an edge-emitting DBR laser; one ECSgrating and one regular DBR grating to make an edge-emitting DBR laser;two ECS gratings with a straight ECS outcoupler grating to make asurface-emitting laser; ECS grating and one regular DBR grating with astraight ECS outcoupler grating to make a surface-emitting laser; twoECS gratings with a “fan-out” ECS outcoupler grating to make asurface-emitting laser; one ECS grating and one regular DBR grating witha “fan-out” ECS outcoupler grating to make a surface-emitting laser; twoECS gratings with a standard grating outcoupler grating to make asurface-emitting laser; one ECS grating and one regular DBR grating witha standard grating outcoupler grating to make a surface-emitting laser;one or more ECS grating output couplers with low back reflection on bothends to make a surface-normal coupled semiconductor optical amplifier(SOA) or optical gain block; or one or more ECS gratings or regular DBRgratings configures as a mirror with high reflectivity and another ECSgrating as an output coupler to make a surface-emitting reflectivesemiconductor optical amplifier (RSOA) or an optical gain block; ahybrid external cavity laser and tunable laser using SOA or RSOA withECS grating output couplers integrated with a waveguide or free spacewavelength control optics; or an enhanced grating for high density andlow loss integration of III/V laser sources for silicon photonicinterconnects.
 11. The grating of claim 1, wherein the optical waveguideprovides lateral optical confinement with a mesa structure and theenhanced grating is on at least one of: (1) the top surface of the mesastructure; (2) one or more lateral surfaces of the mesa structure; or(3) on the surfaces adjacent the mesa structure.
 12. The grating ofclaim 1, wherein an enhanced grating for grating-assisted directionalcouplers; enhanced grating for multiply resonant distributed feedbacklasers; or an enhanced grating for multiplying resonant distributedBragg reflector lasers; an enhanced grating in optical fibers forsampling or detecting light in optical fibers by grating outcouplers; anenhanced gratings in optical fibers for (1) sampling or detecting lightin optical fibers by grating outcouplers operating near the second orderBragg condition; (2) sampling or detecting light in optical fibers bygrating outcouplers operating as distributed Bragg deflectors; (3) tocouple light into optical fibers; a curved, enhanced gratings to makeunstable resonator semiconductor lasers; an enhanced grating to reducethe etch depth for the placement of distributed Bragg reflector gratingsin semiconductor lasers, which results in simplified processing for DBRlasers; an enhanced grating to reduce the etch depth for the placementof distributed Bragg reflector gratings in photonic devices, whichresults in simplified processing for photonic devices; and enhancedgrating to reduce the etch depth for the placement of gratings inphotonic devices, which results in simplified processing for photonicdevices; or an enhanced grating to reduce the etch depth for theplacement of coupling gratings in photonic devices, which results insimplified processing for photonic devices.
 13. The grating of claim 1,wherein the grating comprise a period that is equal to about thewavelength of the light propagating in the optical waveguide to producean outcoupling in about 10 to 50 grating cycles.
 14. The grating ofclaim 1, wherein the grating comprise a period that is equal to aboutone half the wavelength of the light propagating in the opticalwaveguide, and in plane reflectivity of up to about 100% occurs in about5 to 50 grating cycles for light in a typical III-V waveguide.
 15. Thegrating of claim 1, wherein the amorphous or crystalline cover layer isselected from at least one of Si, GaAs, AlGaAs, InP, InGaAsP, GaN,AlGaN, InGaAsPSb, GaP, spin polymers, other column IV, column III-V, orcolumn II-VI semiconductors.
 16. The grating of claim 1, wherein theamorphous or crystalline cover layer is defined further as an amorphousor crystalline high index layer or an amorphous or crystalline low indexcover layer and the amorphous or crystalline cover layer is deposited orformed by at least one of sputtering, vapor phase deposition, plasmaenhanced chemical vapor deposition, vapor phase epitaxy, molecular beamdeposition, molecular beam epitaxy, spin-on, or atomic layer depositionor epitaxial growth over dielectrics through openings in the dielectricto exposed epitaxial material.
 17. The grating of claim 1, wherein theamorphous or crystalline cover layer is defined further as an amorphouslow index cover layer selected from at least one of silicon nitride,polymer, SiO, SiO₂, MgF₂, Al₂O₃, HfO₂, Ta₂O₄₋₅, Sc₂O₃, ZrO₂, TiO₂, CaF₂,ThF₄, ZnS, ZnSe, and other dielectrics.
 18. The grating of claim 1,wherein the amorphous or crystalline cover layer is defined further asan amorphous low index cover layer deposited by at least one ofsputtering, vapor phase deposition, plasma enhanced chemical vapordeposition, vapor phase epitaxy, molecular beam deposition, molecularbeam epitaxy, atomic layer deposition, or by a spin-on processes. 19.The grating of claim 1, wherein the amorphous or crystalline cover layerconverts a grating from a grating region that does not support abound-mode to a grating region that does support a bound-mode.
 20. Thegrating of claim 1, wherein the optical waveguide is defined further ascomprising a non-grating transition waveguide, wherein the non-gratingtransition waveguide comprises a high index amorphous or crystallinecover layer or a low index amorphous or crystalline cover layer thatconverts high loss discontinuity between the waveguide and thetransition waveguide to a low loss discontinuity, and may optionallyfurther comprise a second contrasting amorphous or crystalline coverlayer.
 21. The grating of claim 1, wherein the amorphous or crystallinecover layer when applied over a liner layer converts a grating from agrating region that does not support a bound-mode to a grating regionthat does support a bound-mode.
 22. The grating of claim 1, wherein theridges of the grating extend above the core layer.
 23. The grating ofclaim 1, wherein the thickness of each of the core layer, grating linerlayer, and amorphous or crystalline cover layer are varied to optimizethe ratio of upward coupled radiation to downward coupled radiation orin the upwards or downwards direction.
 24. The grating of claim 1,wherein a period is selected that couples radiation at an anglesufficiently tilted from a surface-normal to reduce or eliminatesecond-order in-plane Bragg reflection.
 25. The grating of claim 1,wherein the optical waveguide further comprises one or more additionalgrooves or ridges each with enhanced coupling strength gratings toprovide a partially reflecting mirror that reduces or cancels asecond-order in-plane Bragg reflection by destructive interference. 26.The grating of claim 1, wherein the optical waveguide further comprisesone or more additional grooves or ridges to provide a partiallyreflecting mirror that reduces or cancels a second-order in-plane Braggreflection by destructive interference.
 27. The grating of claim 1,wherein the optical waveguide further comprises one or more additionalgrooves or ridges that are not covered by at least one of the linerlayer or amorphous or crystalline cover layer to provide a partiallyreflecting mirror that reduces or cancels a second-order in-plane Braggreflection by destructive interference.
 28. The grating of claim 1,wherein the optical waveguide further comprises one or more additionalgrating ridges that are not covered by at least one of the liner layeror the amorphous or crystalline cover layer.
 29. The grating of claim 1,wherein the index of refraction of the liner layer is the range of ˜1.3to ˜1.7, 1.7 to ˜2.2, ˜2.2 to ˜3, or ˜3 to ˜3.8.
 30. The grating ofclaim 1, wherein the amorphous or crystalline cover layer is at leastone of amorphous or crystalline silicon and is defined further as a highindex cover layer that is compatible with silicon processing.
 31. Thegrating of claim 1, wherein the grating period is adapted for use withwavelengths in the range of 0.1 to 0.4, 0.4 to 1.0, 0.5 to 1.1, 0.6 to1.1, and greater than 1.1.
 32. The grating of claim 1, wherein theselection of the materials for the ridges is adapted for use withwavelengths in the range of 0.1 to 0.4, 0.4 to 1.0, 0.5 to 1.1, 0.6 to1.1, and greater than 1.1.
 33. The grating of claim 1, wherein the coreand the ridges are unitary.
 34. The grating of claim 1, wherein thegrating forms at least a portion of an optical waveguide.
 35. Thegrating of claim 1, wherein: a thickness of the liner layer and athickness of the amorphous or crystalline cover layer are determinedusing a Floquet Bloch method, a finite element method, a boundaryelement method or a finite difference time domain method.
 36. Thegrating of claim 1, wherein the liner layer and the cover layer shift afield intensity of the grating into the grating.
 37. The grating ofclaim 1, wherein: n_(liner)<n_(core); n_(liner)<n_(eff), where n_(eff)is an effective index of refraction of the grating; andn_(cover)>n_(eff).
 38. The grating of claim 1, wherein:n_(liner)>n_(core); n_(cover)<n_(core); and n_(cover)≈n_(substrate);where n_(substrate) is an effective index of refraction of thesubstrate.
 39. The grating of claim 1, wherein: the grating has a Figureof Merit (FOM) greater than 1.7; FOM=(a relative permittivitydifference)×(a grating confinement factor); the relative permittivitydifference=(n_(core) ²−n_(liner) ²); and the grating confinementfactor=(a power contained in the grating)/(a total power in thegrating).
 40. The grating of claim 1, wherein: the liner layer has aliner thickness of between 5 and 50 nm; and the amorphous or crystallinecover layer has a cover thickness of between 100 and 300 nm.
 41. Thegrating of claim 1, wherein: the liner layer has a liner thickness ofbetween 1/100^(th) and 1/10^(th) of a free space wavelength divided byan effective index of the grating; and the amorphous or crystallinecover layer has a cover thickness of between ⅕^(th) to 100% of the freespace wavelength divided by the effective index of the grating.
 42. Anoptical waveguide comprising: one or more cladding layers deposited onone or more core layers, wherein the cladding layers comprise arefractive index that is lower than a refractive index of the corelayers (n_(core)); a grating etched into at least one of a claddinglayer, the core, or the cladding and core layers; a liner layer havingan index of refraction (n_(liner)) disposed on the grating; an amorphousor crystalline cover layer disposed the liner layer, wherein theamorphous or crystalline cover layer has an index of refraction(n_(cover)) that is at least one of a lower-index of refraction than theliner layer or a higher index of refraction than the liner layer; and asuperstrate or air layer disposed above the amorphous or crystallinecover layer.
 43. The optical waveguide of claim 42, wherein therefractive index of the cladding layers and core layers are ˜1 to
 2. 44.The optical waveguide of claim 42, wherein the optical waveguide isfurther defined as comprising a grating that has an amorphous orcrystalline cover layer disposed thereon and the index of refraction ofthe amorphous or crystalline cover layer is ˜1 to ˜2, or ˜2 to
 4. 45.The optical waveguide of claim 42, wherein the optical waveguide isfurther defined as comprising a grating with a period that is equal toabout the wavelength of the light propagating in the waveguide toproduce a coupling in 5 to 50 grating cycles.
 46. The optical waveguideof claim 42, wherein the optical waveguide is further defined ascomprising a grating with a period that is equal to about the wavelengthof the light propagating in the waveguide to produce a coupling in 5 to50 microns for light in a typical III-V waveguide at a free spacewavelength of about 1.5 micron.
 47. The optical waveguide of claim 42,wherein: the liner layer has a liner thickness of between 1/100^(th) and1/10^(th) of a free space wavelength divided by an effective index ofthe grating; and the amorphous or crystalline cover layer has a coverthickness of between ⅕^(th) to 100% of the free space wavelength dividedby the effective index of the grating.
 48. A grating, comprising: asubstrate; a core comprising a first material having an index ofrefraction (n_(core)) disposed on the substrate; one or more ridges andone or more grooves comprising the first material formed on the core,wherein the one or more grooves are adjacent to, or between the one ormore ridges, and the ridges and grooves form a grating; a liner layercomprising a second material having an index of refraction (n_(liner))disposed on at least a portion of a grating cycle, wherein the linerlayer has a liner thickness of between 1/100^(th) and 1/10^(th) of afree space wavelength divided by an effective index of the grating; anamorphous or crystalline cover layer comprising a third material havingan index of refraction (n_(cover)) disposed on the liner layer, whereinthe amorphous or crystalline cover layer has a cover thickness ofbetween ⅕^(th) to 100% of the free space wavelength divided by theeffective index of the grating; a superstrate or air layer disposedabove the cover layer; and wherein the third material is not the same asthe first material, n_(core)≠n_(liner) and n_(liner)≠n_(cover).